fix(db): invalidate pool connection when cancelled close fails

Under high-concurrency MySQL load, uvicorn cancels request tasks when
clients disconnect.  If cancellation lands mid-query, session.close()
tries to ROLLBACK on a connection that aiomysql has already marked as
closed — raising InterfaceError("Cancelled during execution") and
leaving the connection checked-out until GC, which the pool then
warns about as a 'non-checked-in connection'.

The old fallback tried sync.rollback() + sync.close(), but those still
go through the async driver and fail the same way on a dead connection.
Replace them with session.sync_session.invalidate(), which just flips
the pool's internal record — no I/O, so it can't be cancelled — and
tells the pool to drop the connection immediately instead of waiting
for garbage collection.

decnet/web/db/sqlmodel_repo.py

@@ -40,28 +40,26 @@ _log = get_logger("db.pool")
 async def _force_close(session: AsyncSession) -> None:
     """Close a session, forcing connection invalidation if clean close fails.
 
-    Shielded from cancellation and catches every exception class including
-    CancelledError.  If session.close() fails (corrupted connection), we
-    invalidate the underlying connection so the pool discards it entirely
-    rather than leaving it checked-out forever.
+    Under cancellation, ``session.close()`` may try to issue a ROLLBACK on
+    a connection that was interrupted mid-query — aiomysql then raises
+    ``InterfaceError("Cancelled during execution")`` and the connection is
+    left checked-out, reported by the pool as ``non-checked-in connection``
+    on GC.  When clean close fails, invalidate the session's connections
+    directly (no I/O, just flips the pool record) so the pool discards
+    them immediately instead of waiting for garbage collection.
     """
     try:
         await asyncio.shield(session.close())
+        return
     except BaseException:
-        # close() failed — connection is likely corrupted.
-        # Try to invalidate the raw connection so the pool drops it.
+        pass
     try:
-            bind = session.get_bind()
-            if hasattr(bind, "dispose"):
-                pass  # don't dispose the whole engine
-            # The sync_session holds the connection record; invalidating
-            # it tells the pool to discard rather than reuse.
-            sync = session.sync_session
-            if sync.is_active:
-                sync.rollback()
-            sync.close()
+        # invalidate() is sync and does no network I/O — safe inside a
+        # cancelled task.  Tells the pool to drop the underlying DBAPI
+        # connection rather than return it for reuse.
+        session.sync_session.invalidate()
     except BaseException:
-            _log.debug("force-close: fallback cleanup failed", exc_info=True)
+        _log.debug("force-close: invalidate failed", exc_info=True)
 
 
 @asynccontextmanager