feat(backend): add operational smoke tooling and safe pricing cleanup endpoints

- add backend README and refresh API map and smoke regression docs
- add full backend smoke regression script
- add admin pricing cleanup preview and dry-run endpoints
- add helper script for test pricing cleanup
- verify typed error contracts, draft flow, publish readiness and preview flows
- verify publish preview retention and clean backend startup behavior

backend/app/api/routes/__init__.py

@@ -1,6 +1,7 @@
 from fastapi import APIRouter
 
 from app.api.routes.admin import router as admin_router
+from app.api.routes.admin_cleanup import router as admin_cleanup_router
 from app.api.routes.audit import router as audit_router
 from app.api.routes.editor import router as editor_router
 from app.api.routes.pricing import router as pricing_router
@@ -22,5 +23,6 @@ router.include_router(pricing_diagnostics_router)
 router.include_router(test_mode_router)
 router.include_router(audit_router)
 router.include_router(admin_router)
+router.include_router(admin_cleanup_router)
 router.include_router(editor_router)
 router.include_router(publish_router)