refactor: introduce Wire dependency injection and interface-based architecture

- Add Google Wire for compile-time dependency injection
- Replace concrete service types with interfaces across handlers
- Remove global state (DB, cache) in favor of constructor injection
- Split monolithic files into focused modules:
  - config: separate files for each config domain
  - dto: converters split by domain (user, post, message, group)
  - cache: separate metrics.go and redis_cache.go
- Introduce unified apperrors package with string-based error codes
- Add transaction support with context-aware repository methods
- Upgrade golang.org/x/crypto from 0.17.0 to 0.26.0

internal/wire/infrastructure.go

@@ -0,0 +1,121 @@
+package wire
+
+import (
+	"log"
+	"time"
+
+	"carrot_bbs/internal/cache"
+	"carrot_bbs/internal/config"
+	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/email"
+	"carrot_bbs/internal/pkg/gorse"
+	"carrot_bbs/internal/pkg/openai"
+	"carrot_bbs/internal/pkg/redis"
+	"carrot_bbs/internal/pkg/s3"
+	"carrot_bbs/internal/pkg/sse"
+	"carrot_bbs/internal/repository"
+
+	"github.com/google/wire"
+	"gorm.io/gorm"
+)
+
+// InfrastructureSet 基础设施 Provider Set
+var InfrastructureSet = wire.NewSet(
+	// 配置
+	ProvideConfig,
+
+	// 数据库
+	ProvideDB,
+
+	// 事务管理器
+	ProvideTransactionManager,
+
+	// Redis 和缓存
+	ProvideRedisClient,
+	ProvideCache,
+
+	// SSE Hub
+	ProvideSSEHub,
+
+	// 外部服务客户端
+	ProvideGorseClient,
+	ProvideOpenAIClient,
+	ProvideEmailClient,
+	ProvideS3Client,
+)
+
+// ProvideConfig 加载配置
+func ProvideConfig() (*config.Config, error) {
+	return config.Load("configs/config.yaml")
+}
+
+// ProvideDB 提供数据库连接
+func ProvideDB(cfg *config.Config) (*gorm.DB, error) {
+	return model.NewDB(&cfg.Database)
+}
+
+// ProvideRedisClient 提供 Redis 客户端
+func ProvideRedisClient(cfg *config.Config) *redis.Client {
+	client, err := redis.New(&cfg.Redis)
+	if err != nil {
+		log.Printf("[WARNING] Failed to connect to Redis: %v, falling back to memory cache", err)
+		return nil
+	}
+	return client
+}
+
+// ProvideCache 提供缓存实例
+func ProvideCache(cfg *config.Config, redisClient *redis.Client) cache.Cache {
+	cache.Configure(cache.Settings{
+		Enabled:         cfg.Cache.Enabled,
+		KeyPrefix:       cfg.Cache.KeyPrefix,
+		DefaultTTL:      time.Duration(cfg.Cache.DefaultTTL) * time.Second,
+		NullTTL:         time.Duration(cfg.Cache.NullTTL) * time.Second,
+		JitterRatio:     cfg.Cache.JitterRatio,
+		PostListTTL:     time.Duration(cfg.Cache.Modules.PostList) * time.Second,
+		ConversationTTL: time.Duration(cfg.Cache.Modules.Conversation) * time.Second,
+		UnreadCountTTL:  time.Duration(cfg.Cache.Modules.UnreadCount) * time.Second,
+		GroupMembersTTL: time.Duration(cfg.Cache.Modules.GroupMembers) * time.Second,
+		DisableFlushDB:  cfg.Cache.DisableFlushDB,
+	})
+
+	// 直接创建 RedisCache 实例，不依赖全局变量
+	if redisClient == nil {
+		log.Println("[Wire] Warning: Redis client is nil, cache will not be available")
+		return nil
+	}
+
+	cacheInstance := cache.NewRedisCache(redisClient)
+	log.Println("[Wire] Initialized Redis cache via dependency injection")
+	return cacheInstance
+}
+
+// ProvideSSEHub 提供 SSE Hub
+func ProvideSSEHub() *sse.Hub {
+	return sse.NewHub()
+}
+
+// ProvideGorseClient 提供 Gorse 客户端
+func ProvideGorseClient(cfg *config.Config) gorse.Client {
+	return gorse.NewClient(gorse.ConfigFromAppConfig(&cfg.Gorse))
+}
+
+// ProvideOpenAIClient 提供 OpenAI 客户端
+func ProvideOpenAIClient(cfg *config.Config) openai.Client {
+	return openai.NewClient(openai.ConfigFromAppConfig(&cfg.OpenAI))
+}
+
+// ProvideEmailClient 提供邮件客户端
+func ProvideEmailClient(cfg *config.Config) email.Client {
+	return email.NewClient(email.ConfigFromAppConfig(&cfg.Email))
+}
+
+// ProvideS3Client 提供 S3 客户端
+func ProvideS3Client(cfg *config.Config) (*s3.Client, error) {
+	return s3.New(&cfg.S3)
+}
+
+// ProvideTransactionManager 提供事务管理器
+func ProvideTransactionManager(db *gorm.DB) repository.TransactionManager {
+	return repository.NewTransactionManager(db)
+}