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/globals.py

@@ -1,14 +1,10 @@
 # globals.py
 import os
-from collections import defaultdict, deque
 import discord
 from apscheduler.schedulers.asyncio import AsyncIOScheduler
 
 scheduler = AsyncIOScheduler()
 
-# Stores last 5 exchanges per user (as deque)
-conversation_history = defaultdict(lambda: deque(maxlen=5))
-
 DISCORD_BOT_TOKEN = os.getenv("DISCORD_BOT_TOKEN")
 
 # Autonomous V2 Debug Mode (set to True to see detailed decision logging)