feat: add flow export/import backend (migration, endpoints, schemas)

Add .rfflow file export/import support:
- Migration 050: import_metadata JSONB column on trees
- GET /trees/{id}/export?format=json|xml endpoint
- POST /trees/import endpoint (creates draft, resolves categories/tags)
- FlowExportEnvelope, FlowImportRequest/Response schemas
- import_metadata field on TreeResponse

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/router.py

@@ -12,6 +12,7 @@ from app.api.endpoints import copilot
 from app.api.endpoints import assistant_chat
 from app.api.endpoints import survey
 from app.api.endpoints import admin_survey
+from app.api.endpoints import tree_transfer
 
 api_router = APIRouter()
 
@@ -48,3 +49,4 @@ api_router.include_router(copilot.router)
 api_router.include_router(assistant_chat.router)
 api_router.include_router(survey.router)
 api_router.include_router(admin_survey.router)
+api_router.include_router(tree_transfer.router)