rebase

app/api/sessions.py

@@ -339,8 +339,18 @@ def _format_sse(evt: dict[str, str]) -> str:
     return "\n".join(lines)
 
 
+from contextlib import asynccontextmanager
+
+
+@asynccontextmanager
 async def _session_scope():
-    """Open a fresh DB session for the background task."""
+    """Open a fresh DB session for the background task.
+
+    Must be used as:  async with _session_scope() as bg_db: ...
+    The @asynccontextmanager decorator is required — without it, an
+    `async def` with `yield` returns an async generator, which does NOT
+    support `async with`.
+    """
     from app.db import get_sessionmaker
 
     sm = get_sessionmaker()