feat(moderation): add manual content review system with three-tier AI moderation

Add admin comment moderation endpoints (single and batch) and introduce a three-tier moderation system (pass/review/block) for content flagged by AI. Content with unclear violations is now held for manual review instead of being auto-rejected. Deleted the legacy audit_service.go in favor of the unified hook-based moderation system.

internal/service/post_service.go

@@ -178,6 +178,10 @@ func (s *postServiceImpl) reviewPostAsync(postID, userID, title, content string,
 	})
 
 	if !result.Approved {
+		if result.NeedsReview {
+			s.invalidatePostCaches(postID)
+			return
+		}
 		if updateErr := s.updateModerationStatusWithRetry(postID, model.PostStatusRejected, result.RejectReason, result.ReviewedBy); updateErr != nil {
 			log.Printf("[WARN] Failed to reject post %s: %v", postID, updateErr)
 		} else {