feat: 添加 Redis Docker 容器编排及三级存储架构

- docker-compose.yml 新增 Redis 服务及 camtalk-net 桥接网络
- 实现 TieredManager 三级存储（L1 内存 → L2 Redis → L3 PostgreSQL）
- config.go 新增 Redis/Persistence 配置类型及环境变量绑定
- 修复 CAMTALK_REDIS_ADDR 环境变量未被 Viper 绑定的问题
- .env.example 更新为三级存储配置并标注开发/部署地址差异

backend/cmd/server/main.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
 
 	"github.com/hhs/camtalk/internal/api"
 	"github.com/hhs/camtalk/internal/auth"
@@ -47,7 +48,7 @@ func main() {
 		"addr", cfg.Server.Addr(),
 	)
 
-	// 初始化存储层（条件初始化 PostgreSQL）
+	// 初始化存储层（三级存储架构：L1 内存 → L2 Redis → L3 PostgreSQL）
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -55,12 +56,17 @@ func main() {
 	var msgRepo store.MessageRepository
 	var sessRepo store.SessionRepository
 
-	if cfg.Storage.Driver == "postgres" {
-		if cfg.Storage.DSN == "" {
-			logger.Log.Fatalw("storage.dsn is required when storage.driver is postgres",
+	// L3: PostgreSQL（冷数据持久化层）
+	dsn := cfg.Storage.Persistence.DSN
+	if dsn == "" {
+		dsn = cfg.Storage.DSN // 兼容旧配置
+	}
+	if cfg.Storage.Persistence.Enabled && cfg.Storage.Persistence.Driver == "postgres" {
+		if dsn == "" {
+			logger.Log.Fatalw("storage.persistence.dsn is required when persistence is enabled",
 				"hint", "set CAMTALK_STORAGE_DSN environment variable")
 		}
-		pool, err := store.NewPostgresPool(ctx, cfg.Storage.DSN)
+		pool, err := store.NewPostgresPool(ctx, dsn)
 		if err != nil {
 			logger.Log.Fatalw("failed to connect to postgres", "error", err)
 		}
@@ -74,27 +80,72 @@ func main() {
 		userRepo = store.NewPgUserRepository(pool)
 		msgRepo = store.NewPgMessageRepository(pool)
 		sessRepo = store.NewPgSessionRepository(pool)
-		logger.Log.Infow("postgres storage initialized", "driver", cfg.Storage.Driver)
+		logger.Log.Infow("L3 PostgreSQL storage initialized", "driver", cfg.Storage.Persistence.Driver)
 	} else {
 		userRepo = store.NewMemUserRepository()
-		logger.Log.Info("using in-memory storage")
+		logger.Log.Info("using in-memory user storage")
 	}
 
-	// 初始化 Session Manager
+	// L2: Redis（热数据分布式会话层）
+	var redisMgr *session.RedisManager
+	if cfg.Storage.Redis.Enabled {
+		rdb := redis.NewClient(&redis.Options{
+			Addr:     cfg.Redis.Addr,
+			Password: cfg.Redis.Password,
+			DB:       cfg.Redis.DB,
+		})
+		// 验证 Redis 连接
+		if err := rdb.Ping(ctx).Err(); err != nil {
+			logger.Log.Fatalw("failed to connect to redis", "error", err)
+		}
+		redisMgr = session.NewRedisManager(
+			rdb,
+			time.Duration(cfg.Session.TTL)*time.Minute,
+			cfg.Session.MaxHistory,
+		)
+		logger.Log.Infow("L2 Redis storage initialized",
+			"addr", cfg.Redis.Addr,
+			"db", cfg.Redis.DB)
+	}
+
+	// 初始化 Session Manager（三级存储）
 	var sessionMgr session.Manager
-	var sessionOpts []session.Option
-	if msgRepo != nil {
-		sessionOpts = append(sessionOpts, session.WithMessageRepository(msgRepo))
+	if cfg.Storage.Redis.Enabled {
+		// L1 + L2 + L3 三级存储
+		var tieredOpts []session.TieredOption
+		if sessRepo != nil {
+			tieredOpts = append(tieredOpts, session.WithTieredSessionRepository(sessRepo))
+		}
+		if msgRepo != nil {
+			tieredOpts = append(tieredOpts, session.WithTieredMessageRepository(msgRepo))
+		}
+		tieredMgr := session.NewTieredManager(
+			time.Duration(cfg.Session.TTL)*time.Minute,
+			cfg.Session.MaxHistory,
+			redisMgr,
+			tieredOpts...,
+		)
+		sessionMgr = tieredMgr
+		defer tieredMgr.Stop()
+		logger.Log.Info("session manager initialized with L1+L2+L3 tiered storage")
+	} else {
+		// L1 + L3 两级存储（无 Redis）
+		var sessionOpts []session.Option
+		if msgRepo != nil {
+			sessionOpts = append(sessionOpts, session.WithMessageRepository(msgRepo))
+		}
+		if sessRepo != nil {
+			sessionOpts = append(sessionOpts, session.WithSessionRepository(sessRepo))
+		}
+		memMgr := session.NewMemoryManager(
+			time.Duration(cfg.Session.TTL)*time.Minute,
+			cfg.Session.MaxHistory,
+			sessionOpts...,
+		)
+		sessionMgr = memMgr
+		defer memMgr.Stop()
+		logger.Log.Info("session manager initialized with L1+L3 storage (Redis disabled)")
 	}
-	if sessRepo != nil {
-		sessionOpts = append(sessionOpts, session.WithSessionRepository(sessRepo))
-	}
-	sessionMgr = session.NewMemoryManager(
-		time.Duration(cfg.Session.TTL)*time.Minute,
-		cfg.Session.MaxHistory,
-		sessionOpts...,
-	)
-	defer sessionMgr.(*session.MemoryManager).Stop()
 
 	// 初始化 AI 服务
 	logger.Log.Infow("initializing AI services",