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/__init__.py

@@ -24,9 +24,21 @@ Usage:
     from app.modules.billing.exceptions import TierLimitExceededException
 """
 
-from app.modules.billing.definition import billing_module, get_billing_module_with_routers
+# Lazy imports to avoid circular dependencies
+# Routers and module definition are imported on-demand
 
 __all__ = [
     "billing_module",
     "get_billing_module_with_routers",
 ]
+
+
+def __getattr__(name: str):
+    """Lazy import to avoid circular dependencies."""
+    if name == "billing_module":
+        from app.modules.billing.definition import billing_module
+        return billing_module
+    elif name == "get_billing_module_with_routers":
+        from app.modules.billing.definition import get_billing_module_with_routers
+        return get_billing_module_with_routers
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")