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/schemas/tree.py

@@ -161,6 +161,7 @@ class TreeResponse(TreeBase):
     created_at: datetime
     updated_at: datetime
     usage_count: int
+    import_metadata: Optional[dict[str, Any]] = None
 
     class Config:
         from_attributes = True