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/handler.go

@@ -0,0 +1,74 @@
+package wire
+
+import (
+	"carrot_bbs/internal/cache"
+	"carrot_bbs/internal/config"
+	"carrot_bbs/internal/handler"
+	"carrot_bbs/internal/pkg/sse"
+	"carrot_bbs/internal/repository"
+	"carrot_bbs/internal/service"
+
+	"github.com/google/wire"
+	"gorm.io/gorm"
+)
+
+// HandlerSet Handler 层 Provider Set
+var HandlerSet = wire.NewSet(
+	// 直接使用构造函数的 Handler
+	handler.NewUserHandler,
+	handler.NewCommentHandler,
+	handler.NewNotificationHandler,
+	handler.NewUploadHandler,
+	handler.NewPushHandler,
+	handler.NewStickerHandler,
+	handler.NewVoteHandler,
+	handler.NewScheduleHandler,
+
+	// 需要特殊处理的 Handler
+	ProvidePostHandler,
+	ProvideMessageHandler,
+	ProvideSystemMessageHandler,
+	ProvideGroupHandler,
+	ProvideGorseHandler,
+)
+
+// ProvidePostHandler 提供帖子处理器
+func ProvidePostHandler(
+	postService service.PostService,
+	userService service.UserService,
+) *handler.PostHandler {
+	return handler.NewPostHandler(postService, userService)
+}
+
+// ProvideMessageHandler 提供消息处理器
+func ProvideMessageHandler(
+	chatService service.ChatService,
+	messageService *service.MessageService,
+	userService service.UserService,
+	groupService service.GroupService,
+	sseHub *sse.Hub,
+) *handler.MessageHandler {
+	return handler.NewMessageHandler(chatService, messageService, userService, groupService, sseHub)
+}
+
+// ProvideSystemMessageHandler 提供系统消息处理器
+func ProvideSystemMessageHandler(
+	systemMsgService service.SystemMessageService,
+	systemNotificationRepo *repository.SystemNotificationRepository,
+	cacheBackend cache.Cache,
+) *handler.SystemMessageHandler {
+	return handler.NewSystemMessageHandler(systemMsgService, systemNotificationRepo, cacheBackend)
+}
+
+// ProvideGroupHandler 提供群组处理器
+func ProvideGroupHandler(
+	groupService service.GroupService,
+	userService service.UserService,
+) *handler.GroupHandler {
+	return handler.NewGroupHandler(groupService, userService)
+}
+
+// ProvideGorseHandler 提供 Gorse 处理器
+func ProvideGorseHandler(cfg *config.Config, db *gorm.DB) *handler.GorseHandler {
+	return handler.NewGorseHandler(cfg.Gorse, db)
+}