refactor: migrate remaining routes to modules and enforce auto-discovery

MIGRATION:
- Delete app/api/v1/vendor/analytics.py (duplicate - analytics module already auto-discovered)
- Move usage routes from app/api/v1/vendor/usage.py to billing module
- Move onboarding routes from app/api/v1/vendor/onboarding.py to marketplace module
- Move features routes to billing module (admin + vendor)
- Move inventory routes to inventory module (admin + vendor)
- Move marketplace/letzshop routes to marketplace module
- Move orders routes to orders module
- Delete legacy letzshop service files (moved to marketplace module)

DOCUMENTATION:
- Add docs/development/migration/module-autodiscovery-migration.md with full migration history
- Update docs/architecture/module-system.md with Entity Auto-Discovery Reference section
- Add detailed sections for each entity type: routes, services, models, schemas, tasks,
  exceptions, templates, static files, locales, configuration

ARCHITECTURE VALIDATION:
- Add MOD-016: Routes must be in modules, not app/api/v1/
- Add MOD-017: Services must be in modules, not app/services/
- Add MOD-018: Tasks must be in modules, not app/tasks/
- Add MOD-019: Schemas must be in modules, not models/schema/
- Update scripts/validate_architecture.py with _validate_legacy_locations method
- Update .architecture-rules/module.yaml with legacy location rules

These rules enforce that all entities must be in self-contained modules.
Legacy locations now trigger ERROR severity violations.

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

app/modules/billing/routes/api/vendor.py

@@ -223,7 +223,9 @@ def get_invoices(
 from app.modules.billing.routes.api.vendor_features import vendor_features_router
 from app.modules.billing.routes.api.vendor_checkout import vendor_checkout_router
 from app.modules.billing.routes.api.vendor_addons import vendor_addons_router
+from app.modules.billing.routes.api.vendor_usage import vendor_usage_router
 
 vendor_router.include_router(vendor_features_router, tags=["vendor-features"])
 vendor_router.include_router(vendor_checkout_router, tags=["vendor-billing"])
 vendor_router.include_router(vendor_addons_router, tags=["vendor-billing-addons"])
+vendor_router.include_router(vendor_usage_router, tags=["vendor-usage"])