refactor(customers): remove legacy route files, fully self-contained

- Delete app/api/v1/admin/customers.py (no backward compatibility)
- Delete app/api/v1/vendor/customers.py (no backward compatibility)
- Remove legacy router aliases from module routes
- Routes now fully self-contained in app/modules/customers/routes/

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

app/modules/customers/routes/admin.py

@@ -120,7 +120,3 @@ def toggle_customer_status(
     )
     db.commit()
     return CustomerMessageResponse(message=result["message"])
-
-
-# Legacy alias for backwards compatibility
-router = admin_router

app/modules/customers/routes/vendor.py

@@ -225,7 +225,3 @@ def get_customer_statistics(
     )
 
     return CustomerStatisticsResponse(**stats)
-
-
-# Legacy alias for backwards compatibility
-router = vendor_router