feat(privacy): add user privacy settings and account deletion system

Add privacy settings allowing users to control visibility of their followers, following, posts, and favorites lists with three visibility levels: everyone, following, and self. Also implement account deletion workflow with 30-day cooldown period.

- Add PrivacySettings model with visibility levels
- Add DTOs for privacy settings and account deletion
- Add repository methods for privacy settings and deletion
- Add service methods to check visibility permissions
- Add handlers for get/update privacy settings and account deletion
- Add new routes for privacy settings and account management
- Add account cleanup service for hard-deleting expired accounts
- Modify login to cancel deletion when user logs in

BREAKING CHANGE: Users with pending_deletion status can no longer log in; logging in now automatically cancels deletion requests.

internal/service/account_cleanup_service.go

@@ -0,0 +1,109 @@
+package service
+
+import (
+	"context"
+	"log"
+	"time"
+
+	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/repository"
+
+	"gorm.io/gorm"
+)
+
+const AccountDeletionCoolDownDays = 90
+
+type AccountCleanupService interface {
+	CleanupExpiredAccounts(ctx context.Context) (int64, error)
+}
+
+type accountCleanupServiceImpl struct {
+	db         *gorm.DB
+	userRepo   repository.UserRepository
+	logService *LogService
+}
+
+func NewAccountCleanupService(
+	db *gorm.DB,
+	userRepo repository.UserRepository,
+	logService *LogService,
+) AccountCleanupService {
+	return &accountCleanupServiceImpl{
+		db:         db,
+		userRepo:   userRepo,
+		logService: logService,
+	}
+}
+
+func (s *accountCleanupServiceImpl) CleanupExpiredAccounts(ctx context.Context) (int64, error) {
+	cutoffTime := time.Now().AddDate(0, 0, -AccountDeletionCoolDownDays)
+
+	users, err := s.userRepo.GetPendingDeletionUsers(cutoffTime)
+	if err != nil {
+		return 0, err
+	}
+
+	if len(users) == 0 {
+		return 0, nil
+	}
+
+	var deletedCount int64
+	for _, user := range users {
+		if err := s.anonymizeAndDeleteUser(ctx, user); err != nil {
+			log.Printf("[AccountCleanup] failed to cleanup user %s: %v", user.ID, err)
+			continue
+		}
+		deletedCount++
+	}
+
+	log.Printf("[AccountCleanup] cleaned up %d expired accounts", deletedCount)
+	return deletedCount, nil
+}
+
+func (s *accountCleanupServiceImpl) anonymizeAndDeleteUser(ctx context.Context, user *model.User) error {
+	deletedUserID := "deleted_user"
+
+	return s.db.Transaction(func(tx *gorm.DB) error {
+		if err := tx.Model(&model.Post{}).Where("user_id = ?", user.ID).Update("user_id", deletedUserID).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Model(&model.Comment{}).Where("user_id = ?", user.ID).Update("user_id", deletedUserID).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Where("follower_id = ? OR following_id = ?", user.ID, user.ID).Delete(&model.Follow{}).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Where("user_id = ?", user.ID).Delete(&model.Favorite{}).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Where("user_id = ?", user.ID).Delete(&model.PostLike{}).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Where("user_id = ?", user.ID).Delete(&model.CommentLike{}).Error; err != nil {
+			return err
+		}
+
+		if err := tx.Unscoped().Delete(&model.User{}, "id = ?", user.ID).Error; err != nil {
+			return err
+		}
+
+		if s.logService != nil {
+			s.logService.OperationLog.RecordOperation(&model.OperationLog{
+				UserID:     user.ID,
+				UserName:   user.Username,
+				Operation:  "account_deleted",
+				Module:     "user",
+				TargetType: "user",
+				TargetID:   user.ID,
+				Status:     "success",
+			})
+		}
+
+		return nil
+	})
+}