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/api/v1/platform/letzshop_vendors.py

@@ -19,7 +19,7 @@ from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
 from app.core.database import get_db
-from app.services.letzshop.vendor_sync_service import LetzshopVendorSyncService
+from app.modules.marketplace.services.letzshop import LetzshopVendorSyncService
 from app.services.platform_signup_service import platform_signup_service
 from app.modules.marketplace.models import LetzshopVendorCache