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/api.py

@@ -1746,9 +1746,9 @@ def get_autonomous_stats():
 
 @app.get("/conversation/{user_id}")
 def get_conversation(user_id: str):
-    if user_id in globals.conversation_history:
-        return {"conversation": list(globals.conversation_history[user_id])}
-    return {"conversation": []}
+    """Get conversation history for a user/channel (uses centralized ConversationHistory)."""
+    messages = conversation_history.get_recent_messages(user_id)
+    return {"conversation": [{"author": author, "content": content, "is_bot": is_bot} for author, content, is_bot in messages]}
 
 # ========== Figurine DM Subscription APIs ==========
 @app.get("/figurines/subscribers")