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

go.mod

@@ -5,6 +5,7 @@ go 1.26.0
 require (
 	github.com/chai2010/webp v1.4.0
 	github.com/gin-gonic/gin v1.12.0
+	github.com/redis/go-redis/v9 v9.20.0
 	github.com/spf13/viper v1.21.0
 	golang.org/x/crypto v0.52.0
 	golang.org/x/image v0.41.0
@@ -16,6 +17,7 @@ require (
 	github.com/bytedance/gopkg v0.1.3 // indirect
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
@@ -50,6 +52,7 @@ require (
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
+	go.uber.org/atomic v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/net v0.54.0 // indirect