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/repository.go

@@ -0,0 +1,29 @@
+package wire
+
+import (
+	"carrot_bbs/internal/repository"
+
+	"github.com/google/wire"
+)
+
+// RepositorySet Repository 层 Provider Set
+var RepositorySet = wire.NewSet(
+	repository.NewUserRepository,
+	repository.NewPostRepository,
+	repository.NewCommentRepository,
+	repository.NewMessageRepository,
+	repository.NewNotificationRepository,
+	repository.NewPushRecordRepository,
+	repository.NewDeviceTokenRepository,
+	repository.NewSystemNotificationRepository,
+	repository.NewGroupRepository,
+	repository.NewStickerRepository,
+	repository.NewVoteRepository,
+	repository.NewScheduleRepository,
+)
+
+// Note: 以下 Repository 返回接口类型而非指针，需要单独处理
+// - ScheduleRepository (repository.ScheduleRepository)
+// - StickerRepository (repository.StickerRepository)
+// - GroupRepository (repository.GroupRepository)
+// 这些已经在各自的 New* 函数中正确返回接口类型，Wire 可以自动处理