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/lb.json

@@ -0,0 +1,99 @@
+{
+  "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."
+  }
+}