perf(webui/session-load-latency) tier3e: fix empty-scene inconclusive prefix read (Greptile P1)

When cached session has no anchor_activity_scenes and the prefix reader
returns None (e.g. sidecar too large for prefix), the empty-scenes path
did not mark _scene_check_inconclusive = True. This meant an inactive
session with matching message counts would return False (cache fresh)
even though disk may have gained its first scene record.

Fix: the empty-scenes else-branch now sets _scene_check_inconclusive
when disk_meta_quick is None, matching the existing behavior in the
cached-scenes branch (line 3173). The code then falls through to the
full metadata comparison instead of serving a stale cache.

discussion_r3548650345

api/models.py

@@ -3180,6 +3180,12 @@ def _cached_session_lags_disk(cached) -> bool:
                 disk_scenes = disk_meta_quick.get('anchor_activity_scenes') or {}
                 if isinstance(disk_scenes, dict) and disk_scenes:
                     return True
+            else:
+                # Prefix read failed (may still succeed via index fallback for
+                # message count). Mark inconclusive so we fall through to the
+                # full metadata comparison instead of returning False with
+                # stale cache. Greptile P1 (discussion_r3548650345).
+                _scene_check_inconclusive = True
         if getattr(cached, 'active_stream_id', None) or getattr(cached, 'pending_user_message', None):
             # Active session: messages may be in flight; fall through to the
             # full check to be safe.