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/service/sticker_service.go

@@ -1,16 +1,18 @@
 package service
 
 import (
-	"carrot_bbs/internal/model"
-	"carrot_bbs/internal/repository"
 	"errors"
 	"net/url"
 	"strings"
+
+	apperrors "carrot_bbs/internal/errors"
+	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/repository"
 )
 
 var (
-	ErrStickerAlreadyExists = &ServiceError{Code: 400, Message: "sticker already exists"}
-	ErrInvalidStickerURL    = &ServiceError{Code: 400, Message: "invalid sticker url"}
+	ErrStickerAlreadyExists = apperrors.ErrStickerAlreadyExists
+	ErrInvalidStickerURL    = apperrors.ErrInvalidStickerURL
 )
 
 // StickerService 自定义表情服务接口