feat: 实现 Redis 会话存储，支持内存/Redis 双模式自动切换

- 新增 go-redis/v9 依赖
- 新增 config.yaml redis 配置段 + RedisConfig 结构体 + REDIS_PASSWORD 环境变量覆盖
- 新增 common/redis.go Redis 客户端初始化
- 完整实现 session/redis_store.go，实现 Store 接口全部 7 个方法
- Redis TTL 自动过期，无须后台清理 goroutine
- deps_core.go 根据 cfg.Redis.Enabled 自动选择 RedisStore/MemoryStore
- router.go + main.go 透传 redisClient 参数

internal/router/deps_core.go

@@ -1,6 +1,7 @@
 package router
 
 import (
+	"log"
 	"time"
 
 	"metazone.cc/metalab/internal/config"
@@ -11,6 +12,7 @@ import (
 	"metazone.cc/metalab/internal/service"
 	"metazone.cc/metalab/internal/session"
 
+	"github.com/redis/go-redis/v9"
 	"gorm.io/gorm"
 )
 
@@ -29,20 +31,29 @@ type dependencies struct {
 	siteSettingCtrl *adminCtrl.SiteSettingController
 }
 
-func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings) (
+func buildDepsCore(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSettings, redisClient *redis.Client) (
 	*repository.UserRepo, *session.Manager, *service.AuthService,
 	*middleware.RateLimiter, *controller.AuthController,
 	*service.AvatarService, *middleware.AuthMiddleware, *adminCtrl.AdminController,
 ) {
 	userRepo := repository.NewUserRepo(db)
 
-	// Session 存储：内存模式（后续可切换为 RedisStore）
-	sessionStore := session.NewMemoryStore(
-		time.Duration(cfg.Session.IdleTimeout)*time.Minute,
-		time.Duration(cfg.Session.RememberTimeout)*time.Minute,
-	)
-	// 启动后台过期清理 goroutine
-	sessionStore.StartCleanup(time.Duration(cfg.Session.CleanupInterval) * time.Second)
+	idleTimeout := time.Duration(cfg.Session.IdleTimeout) * time.Minute
+	rememberTimeout := time.Duration(cfg.Session.RememberTimeout) * time.Minute
+
+	var sessionStore session.Store
+
+	if cfg.Redis.Enabled && redisClient != nil {
+		// Redis 模式（生产环境推荐）
+		sessionStore = session.NewRedisStore(redisClient, idleTimeout, rememberTimeout)
+		log.Println("[Router] 会话存储: Redis")
+	} else {
+		// 内存模式（开发/单实例）
+		memStore := session.NewMemoryStore(idleTimeout, rememberTimeout)
+		memStore.StartCleanup(time.Duration(cfg.Session.CleanupInterval) * time.Second)
+		sessionStore = memStore
+		log.Println("[Router] 会话存储: 内存")
+	}
 
 	sessionMgr := session.NewManager(sessionStore, userRepo, cfg)
 	authService := service.NewAuthService(userRepo, sessionMgr, cfg, siteSettings)