feat: complete billing module self-containment

Migrate billing module routes to self-contained structure:

- routes/api/admin.py - Admin API endpoints
- routes/api/vendor.py - Vendor API endpoints
- routes/pages/ - Page routes (placeholder)
- models/subscription.py - Subscription model (moved)
- schemas/subscription.py - Pydantic schemas (moved)
- locales/ - Translations (en, de, fr, lu)

Removed legacy route files:
- app/modules/billing/routes/admin.py
- app/modules/billing/routes/vendor.py

Updated __init__.py files to use new structure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/billing/locales/de.json

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+{
+  "title": "Abrechnung & Abonnements",
+  "description": "Abonnementstufen, Rechnungshistorie und Zahlungen verwalten",
+  "subscription": {
+    "title": "Abonnement",
+    "current_tier": "Aktuelle Stufe",
+    "status": "Status",
+    "statuses": {
+      "trial": "Testphase",
+      "active": "Aktiv",
+      "past_due": "Überfällig",
+      "cancelled": "Gekündigt",
+      "expired": "Abgelaufen"
+    },
+    "trial_ends": "Testphase endet",
+    "period_ends": "Periode endet",
+    "cancelled_at": "Gekündigt am",
+    "cancellation_reason": "Kündigungsgrund"
+  },
+  "tiers": {
+    "title": "Abonnementstufen",
+    "subtitle": "Preisstufen und Funktionen verwalten",
+    "essential": "Essential",
+    "professional": "Professional",
+    "business": "Business",
+    "enterprise": "Enterprise",
+    "create": "Stufe erstellen",
+    "edit": "Stufe bearbeiten",
+    "features": "Funktionen",
+    "limits": "Limits",
+    "pricing": "Preisgestaltung",
+    "monthly": "Monatlich",
+    "annual": "Jährlich",
+    "per_month": "/Monat",
+    "per_year": "/Jahr",
+    "unlimited": "Unbegrenzt",
+    "orders_per_month": "Bestellungen/Monat",
+    "products_limit": "Produkte",
+    "team_members": "Teammitglieder"
+  },
+  "usage": {
+    "title": "Nutzung",
+    "orders": "Bestellungen",
+    "products": "Produkte",
+    "team": "Teammitglieder",
+    "used": "verwendet",
+    "remaining": "verbleibend",
+    "of": "von"
+  },
+  "invoices": {
+    "title": "Rechnungen",
+    "subtitle": "Rechnungshistorie und Rechnungen",
+    "invoice_number": "Rechnung Nr.",
+    "date": "Datum",
+    "due_date": "Fälligkeitsdatum",
+    "amount": "Betrag",
+    "status": "Status",
+    "download": "PDF herunterladen",
+    "view_online": "Online ansehen",
+    "statuses": {
+      "paid": "Bezahlt",
+      "open": "Offen",
+      "void": "Storniert",
+      "uncollectible": "Uneinbringlich",
+      "draft": "Entwurf"
+    }
+  },
+  "payment": {
+    "title": "Zahlung",
+    "method": "Zahlungsmethode",
+    "add_card": "Karte hinzufügen",
+    "update_card": "Karte aktualisieren",
+    "no_method": "Keine Zahlungsmethode hinterlegt",
+    "card_ending": "Karte endet auf",
+    "expires": "Gültig bis"
+  },
+  "upgrade": {
+    "title": "Upgrade",
+    "compare": "Pläne vergleichen",
+    "select": "Plan auswählen",
+    "current": "Aktueller Plan",
+    "recommended": "Empfohlen"
+  },
+  "messages": {
+    "subscription_updated": "Abonnement erfolgreich aktualisiert",
+    "tier_created": "Stufe erfolgreich erstellt",
+    "tier_updated": "Stufe erfolgreich aktualisiert",
+    "tier_deactivated": "Stufe deaktiviert",
+    "payment_method_updated": "Zahlungsmethode aktualisiert",
+    "subscription_cancelled": "Abonnement gekündigt",
+    "error_loading": "Fehler beim Laden der Abrechnungsinformationen",
+    "error_updating": "Fehler beim Aktualisieren des Abonnements"
+  },
+  "limits": {
+    "orders_exceeded": "Monatliches Bestelllimit erreicht. Upgrade für mehr.",
+    "products_exceeded": "Produktlimit erreicht. Upgrade für mehr.",
+    "team_exceeded": "Teammitgliederlimit erreicht. Upgrade für mehr."
+  }
+}

app/modules/billing/locales/en.json

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+{
+  "title": "Billing & Subscriptions",
+  "description": "Manage subscription tiers, billing history, and payments",
+  "subscription": {
+    "title": "Subscription",
+    "current_tier": "Current Tier",
+    "status": "Status",
+    "statuses": {
+      "trial": "Trial",
+      "active": "Active",
+      "past_due": "Past Due",
+      "cancelled": "Cancelled",
+      "expired": "Expired"
+    },
+    "trial_ends": "Trial ends",
+    "period_ends": "Period ends",
+    "cancelled_at": "Cancelled on",
+    "cancellation_reason": "Cancellation reason"
+  },
+  "tiers": {
+    "title": "Subscription Tiers",
+    "subtitle": "Manage pricing tiers and features",
+    "essential": "Essential",
+    "professional": "Professional",
+    "business": "Business",
+    "enterprise": "Enterprise",
+    "create": "Create Tier",
+    "edit": "Edit Tier",
+    "features": "Features",
+    "limits": "Limits",
+    "pricing": "Pricing",
+    "monthly": "Monthly",
+    "annual": "Annual",
+    "per_month": "/month",
+    "per_year": "/year",
+    "unlimited": "Unlimited",
+    "orders_per_month": "Orders/month",
+    "products_limit": "Products",
+    "team_members": "Team members"
+  },
+  "usage": {
+    "title": "Usage",
+    "orders": "Orders",
+    "products": "Products",
+    "team": "Team Members",
+    "used": "used",
+    "remaining": "remaining",
+    "of": "of"
+  },
+  "invoices": {
+    "title": "Invoices",
+    "subtitle": "Billing history and invoices",
+    "invoice_number": "Invoice #",
+    "date": "Date",
+    "due_date": "Due Date",
+    "amount": "Amount",
+    "status": "Status",
+    "download": "Download PDF",
+    "view_online": "View Online",
+    "statuses": {
+      "paid": "Paid",
+      "open": "Open",
+      "void": "Void",
+      "uncollectible": "Uncollectible",
+      "draft": "Draft"
+    }
+  },
+  "payment": {
+    "title": "Payment",
+    "method": "Payment Method",
+    "add_card": "Add Card",
+    "update_card": "Update Card",
+    "no_method": "No payment method on file",
+    "card_ending": "Card ending in",
+    "expires": "Expires"
+  },
+  "upgrade": {
+    "title": "Upgrade Plan",
+    "compare": "Compare Plans",
+    "select": "Select Plan",
+    "current": "Current Plan",
+    "recommended": "Recommended"
+  },
+  "messages": {
+    "subscription_updated": "Subscription updated successfully",
+    "tier_created": "Tier created successfully",
+    "tier_updated": "Tier updated successfully",
+    "tier_deactivated": "Tier deactivated",
+    "payment_method_updated": "Payment method updated",
+    "subscription_cancelled": "Subscription cancelled",
+    "error_loading": "Error loading billing information",
+    "error_updating": "Error updating subscription"
+  },
+  "limits": {
+    "orders_exceeded": "Monthly order limit reached. Upgrade to continue.",
+    "products_exceeded": "Product limit reached. Upgrade to add more.",
+    "team_exceeded": "Team member limit reached. Upgrade to add more."
+  }
+}

app/modules/billing/locales/fr.json

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+{
+  "title": "Facturation & Abonnements",
+  "description": "Gérer les niveaux d'abonnement, l'historique de facturation et les paiements",
+  "subscription": {
+    "title": "Abonnement",
+    "current_tier": "Niveau actuel",
+    "status": "Statut",
+    "statuses": {
+      "trial": "Essai",
+      "active": "Actif",
+      "past_due": "En retard",
+      "cancelled": "Annulé",
+      "expired": "Expiré"
+    },
+    "trial_ends": "Fin de l'essai",
+    "period_ends": "Fin de période",
+    "cancelled_at": "Annulé le",
+    "cancellation_reason": "Raison de l'annulation"
+  },
+  "tiers": {
+    "title": "Niveaux d'abonnement",
+    "subtitle": "Gérer les tarifs et fonctionnalités",
+    "essential": "Essentiel",
+    "professional": "Professionnel",
+    "business": "Business",
+    "enterprise": "Enterprise",
+    "create": "Créer un niveau",
+    "edit": "Modifier le niveau",
+    "features": "Fonctionnalités",
+    "limits": "Limites",
+    "pricing": "Tarification",
+    "monthly": "Mensuel",
+    "annual": "Annuel",
+    "per_month": "/mois",
+    "per_year": "/an",
+    "unlimited": "Illimité",
+    "orders_per_month": "Commandes/mois",
+    "products_limit": "Produits",
+    "team_members": "Membres d'équipe"
+  },
+  "usage": {
+    "title": "Utilisation",
+    "orders": "Commandes",
+    "products": "Produits",
+    "team": "Membres d'équipe",
+    "used": "utilisé",
+    "remaining": "restant",
+    "of": "sur"
+  },
+  "invoices": {
+    "title": "Factures",
+    "subtitle": "Historique de facturation et factures",
+    "invoice_number": "Facture N°",
+    "date": "Date",
+    "due_date": "Date d'échéance",
+    "amount": "Montant",
+    "status": "Statut",
+    "download": "Télécharger PDF",
+    "view_online": "Voir en ligne",
+    "statuses": {
+      "paid": "Payée",
+      "open": "Ouverte",
+      "void": "Annulée",
+      "uncollectible": "Irrécouvrable",
+      "draft": "Brouillon"
+    }
+  },
+  "payment": {
+    "title": "Paiement",
+    "method": "Moyen de paiement",
+    "add_card": "Ajouter une carte",
+    "update_card": "Modifier la carte",
+    "no_method": "Aucun moyen de paiement enregistré",
+    "card_ending": "Carte se terminant par",
+    "expires": "Expire"
+  },
+  "upgrade": {
+    "title": "Mettre à niveau",
+    "compare": "Comparer les plans",
+    "select": "Sélectionner le plan",
+    "current": "Plan actuel",
+    "recommended": "Recommandé"
+  },
+  "messages": {
+    "subscription_updated": "Abonnement mis à jour avec succès",
+    "tier_created": "Niveau créé avec succès",
+    "tier_updated": "Niveau mis à jour avec succès",
+    "tier_deactivated": "Niveau désactivé",
+    "payment_method_updated": "Moyen de paiement mis à jour",
+    "subscription_cancelled": "Abonnement annulé",
+    "error_loading": "Erreur lors du chargement des informations de facturation",
+    "error_updating": "Erreur lors de la mise à jour de l'abonnement"
+  },
+  "limits": {
+    "orders_exceeded": "Limite mensuelle de commandes atteinte. Passez à un niveau supérieur.",
+    "products_exceeded": "Limite de produits atteinte. Passez à un niveau supérieur.",
+    "team_exceeded": "Limite de membres d'équipe atteinte. Passez à un niveau supérieur."
+  }
+}

app/modules/billing/locales/lb.json

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+{
+  "title": "Rechnung & Abonnementer",
+  "description": "Abonnementstufe, Rechnungshistorik an Zuelungen verwalten",
+  "subscription": {
+    "title": "Abonnement",
+    "current_tier": "Aktuell Stuf",
+    "status": "Status",
+    "statuses": {
+      "trial": "Testphas",
+      "active": "Aktiv",
+      "past_due": "Iwwerfälleg",
+      "cancelled": "Gekënnegt",
+      "expired": "Ofgelaf"
+    },
+    "trial_ends": "Testphas endet",
+    "period_ends": "Period endet",
+    "cancelled_at": "Gekënnegt den",
+    "cancellation_reason": "Kënnegungsgrond"
+  },
+  "tiers": {
+    "title": "Abonnementstufen",
+    "subtitle": "Präisstufen a Funktiounen verwalten",
+    "essential": "Essential",
+    "professional": "Professional",
+    "business": "Business",
+    "enterprise": "Enterprise",
+    "create": "Stuf erstellen",
+    "edit": "Stuf beaarbechten",
+    "features": "Funktiounen",
+    "limits": "Limiten",
+    "pricing": "Präisgestaltung",
+    "monthly": "Monatlech",
+    "annual": "Jäerlech",
+    "per_month": "/Mount",
+    "per_year": "/Joer",
+    "unlimited": "Onbegrenzt",
+    "orders_per_month": "Bestellungen/Mount",
+    "products_limit": "Produkter",
+    "team_members": "Teammemberen"
+  },
+  "usage": {
+    "title": "Notzung",
+    "orders": "Bestellungen",
+    "products": "Produkter",
+    "team": "Teammemberen",
+    "used": "benotzt",
+    "remaining": "iwwreg",
+    "of": "vun"
+  },
+  "invoices": {
+    "title": "Rechnungen",
+    "subtitle": "Rechnungshistorik a Rechnungen",
+    "invoice_number": "Rechnung Nr.",
+    "date": "Datum",
+    "due_date": "Fällegkeetsdatum",
+    "amount": "Betrag",
+    "status": "Status",
+    "download": "PDF eroflueden",
+    "view_online": "Online kucken",
+    "statuses": {
+      "paid": "Bezuelt",
+      "open": "Oppen",
+      "void": "Stornéiert",
+      "uncollectible": "Onabtreidbar",
+      "draft": "Entworf"
+    }
+  },
+  "payment": {
+    "title": "Zuelung",
+    "method": "Zuelungsmethod",
+    "add_card": "Kaart dobäisetzen",
+    "update_card": "Kaart aktualiséieren",
+    "no_method": "Keng Zuelungsmethod hannerlued",
+    "card_ending": "Kaart endet op",
+    "expires": "Gëlteg bis"
+  },
+  "upgrade": {
+    "title": "Upgrade",
+    "compare": "Pläng vergläichen",
+    "select": "Plang auswielen",
+    "current": "Aktuellen Plang",
+    "recommended": "Recommandéiert"
+  },
+  "messages": {
+    "subscription_updated": "Abonnement erfollegräich aktualiséiert",
+    "tier_created": "Stuf erfollegräich erstallt",
+    "tier_updated": "Stuf erfollegräich aktualiséiert",
+    "tier_deactivated": "Stuf deaktivéiert",
+    "payment_method_updated": "Zuelungsmethod aktualiséiert",
+    "subscription_cancelled": "Abonnement gekënnegt",
+    "error_loading": "Feeler beim Lueden vun de Rechnungsinformatiounen",
+    "error_updating": "Feeler beim Aktualiséieren vum Abonnement"
+  },
+  "limits": {
+    "orders_exceeded": "Monatlech Bestellungslimit erreecht. Upgrade fir méi.",
+    "products_exceeded": "Produktlimit erreecht. Upgrade fir méi.",
+    "team_exceeded": "Teammemberlimit erreecht. Upgrade fir méi."
+  }
+}