refactor: consolidate conversation_history to ConversationHistory class

Remove legacy globals.conversation_history (defaultdict of deques) and
route all callers through utils.conversation_history.ConversationHistory:

- globals.py: remove conversation_history + unused collections imports
- llm.py: remove backward-compat dual-write to legacy system
- api.py: /conversation/{user_id} now reads from ConversationHistory
- actions.py: reset_conversation uses clear_channel()
- figurine_notifier.py: use add_message() instead of buggy setdefault()
- bipolar_mode.py: fix clear_history -> clear_channel (was AttributeError
  silently swallowed by bare except), fix bare except -> except Exception

bot/utils/bipolar_mode.py

@@ -1034,8 +1034,8 @@ async def run_argument(channel: discord.TextChannel, client, trigger_context: st
                     
                     # Clean up argument conversation history
                     try:
-                        conversation_history.clear_history(argument_user_id)
-                    except:
+                        conversation_history.clear_channel(argument_user_id)
+                    except Exception:
                         pass  # History cleanup is not critical
                     
                     end_argument(channel_id)