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/group_service.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"carrot_bbs/internal/cache"
+	apperrors "carrot_bbs/internal/errors"
 	"carrot_bbs/internal/model"
 	"carrot_bbs/internal/pkg/sse"
 	"carrot_bbs/internal/pkg/utils"
@@ -23,25 +24,26 @@ const (
 	GroupCacheJitter    = 0.1
 )
 
-// 群组服务错误定义
+// 群组服务错误定义 - 使用统一的 AppError
 var (
-	ErrGroupNotFound        = &ServiceError{Code: 404, Message: "群组不存在"}
-	ErrNotGroupMember       = &ServiceError{Code: 403, Message: "不是群成员"}
-	ErrNotGroupAdmin        = &ServiceError{Code: 403, Message: "不是群管理员"}
-	ErrNotGroupOwner        = &ServiceError{Code: 403, Message: "不是群主"}
-	ErrGroupFull            = &ServiceError{Code: 400, Message: "群已满"}
-	ErrAlreadyMember        = &ServiceError{Code: 400, Message: "已经是群成员"}
-	ErrCannotRemoveOwner    = &ServiceError{Code: 400, Message: "不能移除群主"}
-	ErrCannotMuteOwner      = &ServiceError{Code: 400, Message: "不能禁言群主"}
-	ErrMuted                = &ServiceError{Code: 403, Message: "你已被禁言"}
-	ErrMuteAllEnabled       = &ServiceError{Code: 403, Message: "全员禁言中"}
-	ErrCannotJoin           = &ServiceError{Code: 400, Message: "该群不允许加入"}
-	ErrJoinRequestPending   = &ServiceError{Code: 400, Message: "加群申请已提交"}
-	ErrGroupRequestNotFound = &ServiceError{Code: 404, Message: "加群请求不存在"}
-	ErrGroupRequestHandled  = &ServiceError{Code: 400, Message: "该加群请求已处理"}
-	ErrNotRequestTarget     = &ServiceError{Code: 400, Message: "不是邀请目标用户"}
-	ErrNoEligibleInvitee    = &ServiceError{Code: 400, Message: "没有可邀请的用户"}
-	ErrNotMutualFollow      = &ServiceError{Code: 400, Message: "仅支持邀请互相关注用户"}
+	ErrGroupNotFound        = apperrors.ErrGroupNotFound
+	ErrNotGroupMember       = apperrors.ErrNotGroupMember
+	ErrNotGroupAdmin        = apperrors.ErrNotGroupAdmin
+	ErrNotGroupOwner        = apperrors.ErrNotGroupOwner
+	ErrGroupFull            = apperrors.ErrGroupFull
+	ErrAlreadyMember        = apperrors.ErrAlreadyGroupMember
+	ErrCannotRemoveOwner    = apperrors.ErrCannotRemoveOwner
+	ErrCannotMuteOwner      = apperrors.ErrCannotMuteOwner
+	ErrMuted                = apperrors.ErrMuted
+	ErrMuteAllEnabled       = apperrors.ErrMuteAllEnabled
+	ErrCannotJoin           = apperrors.ErrCannotJoin
+	ErrJoinRequestPending   = apperrors.ErrJoinRequestPending
+	ErrGroupRequestNotFound = apperrors.ErrGroupRequestNotFound
+	ErrGroupRequestHandled  = apperrors.ErrGroupRequestHandled
+	ErrNotRequestTarget     = apperrors.ErrNotRequestTarget
+	ErrNoEligibleInvitee    = apperrors.ErrNoEligibleInvitee
+	ErrNotMutualFollow      = apperrors.ErrNotMutualFollow
+	ErrInviteNotAccepted    = apperrors.ErrInviteNotAccepted
 )
 
 // GroupService 群组服务接口
@@ -104,7 +106,7 @@ type groupService struct {
 }
 
 // NewGroupService 创建群组服务
-func NewGroupService(db *gorm.DB, groupRepo repository.GroupRepository, userRepo *repository.UserRepository, messageRepo *repository.MessageRepository, sseHub *sse.Hub) GroupService {
+func NewGroupService(db *gorm.DB, groupRepo repository.GroupRepository, userRepo *repository.UserRepository, messageRepo *repository.MessageRepository, sseHub *sse.Hub, cacheBackend cache.Cache) GroupService {
 	return &groupService{
 		db:          db,
 		groupRepo:   groupRepo,
@@ -113,7 +115,7 @@ func NewGroupService(db *gorm.DB, groupRepo repository.GroupRepository, userRepo
 		requestRepo: repository.NewGroupJoinRequestRepository(db),
 		notifyRepo:  repository.NewSystemNotificationRepository(db),
 		sseHub:      sseHub,
-		cache:       cache.GetCache(),
+		cache:       cacheBackend,
 	}
 }
 
@@ -937,7 +939,7 @@ func (s *groupService) SetGroupAddRequest(userID string, flag string, approve bo
 	if req.RequestType == model.GroupJoinRequestTypeInvite {
 		if req.ReviewerID == "" {
 			// 被邀请人还未同意，管理员不能提前审批
-			return &ServiceError{Code: 400, Message: "被邀请人尚未同意邀请，无法审批"}
+			return ErrInviteNotAccepted
 		}
 		if req.ReviewerID != req.TargetUserID {
 			// ReviewerID 不是被邀请人，说明已经被其他人处理过