cleanup: remove dead code + deduplicate GPU state reads

Dead code removed:
- globals.py: GUILD_SETTINGS (empty dict, zero consumers)
- config.py: unused 'import os'
- config_manager.py: unused 'import os' and 'Union'
- server_manager.py: duplicate 'from datetime import datetime, timedelta'

GPU deduplication:
- get_current_gpu_url() now delegates to config_manager.get_gpu()
- get_gpu_status() endpoint now delegates to config_manager.get_gpu()
- Both previously re-read memory/gpu_state.json directly

bot/globals.py

@@ -6,8 +6,6 @@ from apscheduler.schedulers.asyncio import AsyncIOScheduler
 
 scheduler = AsyncIOScheduler()
 
-GUILD_SETTINGS = {}
-
 # Stores last 5 exchanges per user (as deque)
 conversation_history = defaultdict(lambda: deque(maxlen=5))