Add Redis caching, email refresh button, optimize page loading

- Add Redis service (docker-compose) for caching accounts, messages, payment status
- Cache accounts list (5min), messages (3min), payment status (10min)
- Auto-invalidate cache on import/delete/payment-check/note-update
- Add refresh button to email list panel (force re-fetch from IMAP)
- Messages API supports refresh=true param to bypass cache
- New cache.py module with RedisCache class

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -10,6 +10,9 @@ SERVER_PORT=5000
 # 服务器主机（默认0.0.0.0，接受所有连接）
 SERVER_HOST=0.0.0.0
 
+# Redis缓存地址（Docker内部自动连接，无需修改）
+REDIS_URL=redis://redis:6379/0
+
 # 时区设置
 TZ=Asia/Shanghai