perf(MAT): fire-and-forget auto-rename + harden memory-extract None response

- `_auto_rename_room` now runs as a tracked bg task. Title generation
  latency no longer affects when the handler returns and frees the
  room lock, so users can fire the next message sooner.
- Memory extraction guards against providers returning `None` for
  `choices[0].message.content` (observed in logs: AttributeError on
  .strip). Logs once and returns cleanly instead of raising.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bot.py

@@ -1974,7 +1974,13 @@ class Bot:
                 ],
                 max_tokens=300,
             )
-            raw = resp.choices[0].message.content.strip()
+            raw_content = resp.choices[0].message.content
+            if not raw_content:
+                # Some providers return None content when the model decides to emit nothing
+                # (e.g. safety filter, empty JSON array would still be a string though).
+                logger.info("Memory extraction: empty model response, nothing to store")
+                return
+            raw = raw_content.strip()
             logger.info("Memory extraction raw response: %s", raw[:200])
 
             if raw.startswith("```"):
@@ -3306,12 +3312,18 @@ class Bot:
                 self._bg_tasks.add(task)
                 task.add_done_callback(self._bg_tasks.discard)
 
-            # Auto-rename: only for group rooms with explicit opt-in (not DMs)
+            # Auto-rename: only for group rooms with explicit opt-in (not DMs).
+            # Fire-and-forget so the title-generation LLM call doesn't gate
+            # the turn return path.
             if room.room_id in self.auto_rename_rooms:
                 last_rename = self.renamed_rooms.get(room.room_id, 0)
                 gap_seconds = time.time() - last_rename if last_rename else float("inf")
                 if gap_seconds > 300:
-                    await self._auto_rename_room(room, user_message, reply)
+                    rename_task = asyncio.create_task(
+                        self._auto_rename_room(room, user_message, reply)
+                    )
+                    self._bg_tasks.add(rename_task)
+                    rename_task.add_done_callback(self._bg_tasks.discard)
 
             return reply
         except _openai.APIStatusError as e: