fix: add thumbnail cleanup on archive deletion and diagnostic scripts

- Fix archive deletion to also clean up associated thumbnails
- Add cleanup-thumbnails.js script to remove temporary and orphaned thumbnails
- Add diagnose-thumbnails.js script to troubleshoot thumbnail serving issues
- Update README with documentation for new scripts

This prevents thumbnail accumulation when events are deleted and helps
diagnose why thumbnails might not be showing despite existing on disk.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/routes/adminArchives.js

@@ -363,12 +363,29 @@ router.delete('/:id', adminAuth, async (req, res) => {
     // Delete archive file if exists
     if (archive.archive_path) {
       try {
-        await fs.unlink(archive.archive_path);
+        const storagePath = process.env.STORAGE_PATH || path.join(__dirname, '../../../storage');
+        const fullArchivePath = path.join(storagePath, archive.archive_path);
+        await fs.unlink(fullArchivePath);
       } catch (error) {
         console.error('Failed to delete archive file:', error);
       }
     }
 
+    // Delete thumbnails for this event
+    const photos = await db('photos').where('event_id', req.params.id).select('thumbnail_path');
+    const storagePath = process.env.STORAGE_PATH || path.join(__dirname, '../../../storage');
+    
+    for (const photo of photos) {
+      if (photo.thumbnail_path) {
+        try {
+          const thumbPath = path.join(storagePath, photo.thumbnail_path.replace(/^\//, ''));
+          await fs.unlink(thumbPath);
+        } catch (error) {
+          // Ignore errors - thumbnail might already be deleted
+        }
+      }
+    }
+
     // Delete from database (cascade will delete photos and logs)
     await db('events').where('id', req.params.id).delete();