fix: address security vulnerabilities and add image attachments

- Fix type confusion vulnerability in comment content updates
- Validate pagination offsets to prevent negative values
- Validate timestamp is a valid number before parsing
- Exclude guestEmail from comment API responses for privacy
- Fix TypeScript error in audio upload route
- Add image attachment support for comments with upload API
- Update admin dashboard to track image attachments
- Rename cleanup functions to handle both voice and image media

prisma/schema.prisma

@@ -263,6 +263,9 @@ model Comment {
   voiceUrl      String?     // URL to voice recording file
   voiceDuration Float?      // Duration of voice recording in seconds
   
+  // Image attachment (optional)
+  imageUrl      String?     // URL to uploaded image file
+  
   // Threading
   parentId      String?
   parent        Comment?    @relation("CommentReplies", fields: [parentId], references: [id], onDelete: Cascade)