feat: add Redis support for distributed passkey sessions

- Implement SessionStore interface for pluggable storage backends
- Add redisStore for distributed environment
- Add memoryStore as fallback for single instance
- Update router to initialize Redis client when configured
- Update .env.example with Redis documentation

.env.example

@@ -24,7 +24,7 @@ DB_DSN=
 DB_MAX_OPEN_CONNS=10
 DB_MAX_IDLE_CONNS=5
 
-# --- Redis 配置（可选）---
+# --- Redis 配置（可选，用于分布式 passkey session）---
 # REDIS_HOST=localhost
 # REDIS_PORT=6379
 # REDIS_PASSWORD=