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/schemas/subscription.py

@@ -0,0 +1,209 @@
+# app/modules/billing/schemas/subscription.py
+"""
+Pydantic schemas for subscription operations.
+
+Supports subscription management and tier limit checks.
+"""
+
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+# ============================================================================
+# Tier Information Schemas
+# ============================================================================
+
+
+class TierFeatures(BaseModel):
+    """Features included in a tier."""
+
+    letzshop_sync: bool = True
+    inventory_basic: bool = True
+    inventory_locations: bool = False
+    inventory_purchase_orders: bool = False
+    invoice_lu: bool = True
+    invoice_eu_vat: bool = False
+    invoice_bulk: bool = False
+    customer_view: bool = True
+    customer_export: bool = False
+    analytics_dashboard: bool = False
+    accounting_export: bool = False
+    api_access: bool = False
+    automation_rules: bool = False
+    team_roles: bool = False
+    white_label: bool = False
+    multi_vendor: bool = False
+    custom_integrations: bool = False
+    sla_guarantee: bool = False
+    dedicated_support: bool = False
+
+
+class TierLimits(BaseModel):
+    """Limits for a subscription tier."""
+
+    orders_per_month: int | None = Field(None, description="None = unlimited")
+    products_limit: int | None = Field(None, description="None = unlimited")
+    team_members: int | None = Field(None, description="None = unlimited")
+    order_history_months: int | None = Field(None, description="None = unlimited")
+
+
+class TierInfo(BaseModel):
+    """Full tier information."""
+
+    code: str
+    name: str
+    price_monthly_cents: int
+    price_annual_cents: int | None
+    limits: TierLimits
+    features: list[str]
+
+
+# ============================================================================
+# Subscription Schemas
+# ============================================================================
+
+
+class SubscriptionCreate(BaseModel):
+    """Schema for creating a subscription (admin/internal use)."""
+
+    tier: str = Field(default="essential", pattern="^(essential|professional|business|enterprise)$")
+    is_annual: bool = False
+    trial_days: int = Field(default=14, ge=0, le=30)
+
+
+class SubscriptionUpdate(BaseModel):
+    """Schema for updating a subscription."""
+
+    tier: str | None = Field(None, pattern="^(essential|professional|business|enterprise)$")
+    status: str | None = Field(None, pattern="^(trial|active|past_due|cancelled|expired)$")
+    is_annual: bool | None = None
+    custom_orders_limit: int | None = None
+    custom_products_limit: int | None = None
+    custom_team_limit: int | None = None
+
+
+class SubscriptionResponse(BaseModel):
+    """Schema for subscription response."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    vendor_id: int
+    tier: str
+    status: str
+
+    period_start: datetime
+    period_end: datetime
+    is_annual: bool
+
+    trial_ends_at: datetime | None
+    orders_this_period: int
+    orders_limit_reached_at: datetime | None
+
+    # Effective limits (with custom overrides applied)
+    orders_limit: int | None
+    products_limit: int | None
+    team_members_limit: int | None
+
+    # Computed properties
+    is_active: bool
+    is_trial: bool
+    trial_days_remaining: int | None
+
+    created_at: datetime
+    updated_at: datetime
+
+
+class SubscriptionUsage(BaseModel):
+    """Current subscription usage statistics."""
+
+    orders_used: int
+    orders_limit: int | None
+    orders_remaining: int | None
+    orders_percent_used: float | None
+
+    products_used: int
+    products_limit: int | None
+    products_remaining: int | None
+    products_percent_used: float | None
+
+    team_members_used: int
+    team_members_limit: int | None
+    team_members_remaining: int | None
+    team_members_percent_used: float | None
+
+
+class UsageSummary(BaseModel):
+    """Usage summary for billing page display."""
+
+    orders_this_period: int
+    orders_limit: int | None
+    orders_remaining: int | None
+
+    products_count: int
+    products_limit: int | None
+    products_remaining: int | None
+
+    team_count: int
+    team_limit: int | None
+    team_remaining: int | None
+
+
+class SubscriptionStatusResponse(BaseModel):
+    """Subscription status with usage and limits."""
+
+    subscription: SubscriptionResponse
+    usage: SubscriptionUsage
+    tier_info: TierInfo
+
+
+# ============================================================================
+# Limit Check Schemas
+# ============================================================================
+
+
+class LimitCheckResult(BaseModel):
+    """Result of a limit check."""
+
+    allowed: bool
+    limit: int | None
+    current: int
+    remaining: int | None
+    message: str | None = None
+
+
+class CanCreateOrderResponse(BaseModel):
+    """Response for order creation check."""
+
+    allowed: bool
+    orders_this_period: int
+    orders_limit: int | None
+    message: str | None = None
+
+
+class CanAddProductResponse(BaseModel):
+    """Response for product addition check."""
+
+    allowed: bool
+    products_count: int
+    products_limit: int | None
+    message: str | None = None
+
+
+class CanAddTeamMemberResponse(BaseModel):
+    """Response for team member addition check."""
+
+    allowed: bool
+    team_count: int
+    team_limit: int | None
+    message: str | None = None
+
+
+class FeatureCheckResponse(BaseModel):
+    """Response for feature check."""
+
+    feature: str
+    enabled: bool
+    tier_required: str | None = None
+    message: str | None = None