fix: break Discord typing loop on persistent HTTP failure

The typing indicator loop catches all exceptions with bare
except/pass, so a permanent HTTP failure (client closed, auth
error, etc.) causes the loop to spin every 8 seconds doing
nothing until the channel is explicitly stopped.

Log the error and exit the loop instead, letting the task
clean up naturally.

nanobot/channels/discord.py

@@ -285,8 +285,11 @@ class DiscordChannel(BaseChannel):
             while self._running:
                 try:
                     await self._http.post(url, headers=headers)
-                except Exception:
-                    pass
+                except asyncio.CancelledError:
+                    return
+                except Exception as e:
+                    logger.debug("Discord typing indicator failed for {}: {}", channel_id, e)
+                    return
                 await asyncio.sleep(8)
 
         self._typing_tasks[channel_id] = asyncio.create_task(typing_loop())