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/server_manager.py

@@ -12,7 +12,6 @@ from apscheduler.triggers.interval import IntervalTrigger
 from apscheduler.triggers.cron import CronTrigger
 from apscheduler.triggers.date import DateTrigger
 import random
-from datetime import datetime, timedelta
 from utils.logger import get_logger
 
 logger = get_logger('server')