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/llm.py

@@ -475,10 +475,6 @@ Please respond in a way that reflects this emotional tone.{pfp_context}"""
                             is_bot=True
                         )
                     
-                    # Also save to legacy globals for backward compatibility (skip error messages)
-                    if user_prompt and user_prompt.strip() and reply and reply.strip() and reply != "Someone tell Koko-nii there is a problem with my AI.":
-                        globals.conversation_history[user_id].append((user_prompt, reply))
-                    
                     return reply
                 else:
                     error_text = await response.text()