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/dto/group_converter.go

@@ -0,0 +1,96 @@
+package dto
+
+import (
+	"carrot_bbs/internal/model"
+)
+
+// ==================== Group 转换 ====================
+
+// GroupToResponse 将Group转换为GroupResponse
+func GroupToResponse(group *model.Group) *GroupResponse {
+	if group == nil {
+		return nil
+	}
+	return &GroupResponse{
+		ID:          group.ID,
+		Name:        group.Name,
+		Avatar:      group.Avatar,
+		Description: group.Description,
+		OwnerID:     group.OwnerID,
+		MemberCount: group.MemberCount,
+		MaxMembers:  group.MaxMembers,
+		JoinType:    int(group.JoinType),
+		MuteAll:     group.MuteAll,
+		CreatedAt:   FormatTime(group.CreatedAt),
+	}
+}
+
+// GroupsToResponse 将Group列表转换为GroupResponse列表
+func GroupsToResponse(groups []model.Group) []*GroupResponse {
+	result := make([]*GroupResponse, 0, len(groups))
+	for i := range groups {
+		result = append(result, GroupToResponse(&groups[i]))
+	}
+	return result
+}
+
+// GroupMemberToResponse 将GroupMember转换为GroupMemberResponse
+func GroupMemberToResponse(member *model.GroupMember) *GroupMemberResponse {
+	if member == nil {
+		return nil
+	}
+	return &GroupMemberResponse{
+		ID:       member.ID,
+		GroupID:  member.GroupID,
+		UserID:   member.UserID,
+		Role:     member.Role,
+		Nickname: member.Nickname,
+		Muted:    member.Muted,
+		JoinTime: FormatTime(member.JoinTime),
+	}
+}
+
+// GroupMemberToResponseWithUser 将GroupMember转换为GroupMemberResponse（包含用户信息）
+func GroupMemberToResponseWithUser(member *model.GroupMember, user *model.User) *GroupMemberResponse {
+	if member == nil {
+		return nil
+	}
+	resp := GroupMemberToResponse(member)
+	if user != nil {
+		resp.User = ConvertUserToResponse(user)
+	}
+	return resp
+}
+
+// GroupMembersToResponse 将GroupMember列表转换为GroupMemberResponse列表
+func GroupMembersToResponse(members []model.GroupMember) []*GroupMemberResponse {
+	result := make([]*GroupMemberResponse, 0, len(members))
+	for i := range members {
+		result = append(result, GroupMemberToResponse(&members[i]))
+	}
+	return result
+}
+
+// GroupAnnouncementToResponse 将GroupAnnouncement转换为GroupAnnouncementResponse
+func GroupAnnouncementToResponse(announcement *model.GroupAnnouncement) *GroupAnnouncementResponse {
+	if announcement == nil {
+		return nil
+	}
+	return &GroupAnnouncementResponse{
+		ID:        announcement.ID,
+		GroupID:   announcement.GroupID,
+		Content:   announcement.Content,
+		AuthorID:  announcement.AuthorID,
+		IsPinned:  announcement.IsPinned,
+		CreatedAt: FormatTime(announcement.CreatedAt),
+	}
+}
+
+// GroupAnnouncementsToResponse 将GroupAnnouncement列表转换为GroupAnnouncementResponse列表
+func GroupAnnouncementsToResponse(announcements []model.GroupAnnouncement) []*GroupAnnouncementResponse {
+	result := make([]*GroupAnnouncementResponse, 0, len(announcements))
+	for i := range announcements {
+		result = append(result, GroupAnnouncementToResponse(&announcements[i]))
+	}
+	return result
+}