refactor: tighten backend, flows, frontend, tests; push flows to Langflow

backend/app/db.py

@@ -44,6 +44,11 @@ def get_session() -> Iterator[Session]:
         yield session
 
 
+def get_db() -> Iterator[Session]:
+    """Single FastAPI DB dependency (all routers import this)."""
+    yield from get_session()
+
+
 def create_all(url: str | None = None) -> None:
     """Create tables directly (dev convenience; migrations own prod schema)."""
     SQLModel.metadata.create_all(get_engine(url))