fix(mail): frontend filename decoding for attachments + backend robust decoding

- Frontend: decodeMimeHeader() on attachment filenames in MailDetail for display
- Backend: _decode_mime_filename() helper with Q-encoding fallback for edge cases
- Attachments now visible in mail detail UI (was 0 records, now 44+)
- Notifications table updated_at column added via migration 0020

frontend/src/components/mail/MailDetail.tsx

@@ -160,7 +160,7 @@ export function MailDetail({
                     <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z" />
                   </svg>
                   <div className="flex-1 min-w-0">
-                    <p className="text-sm text-secondary-800 truncate">{att.filename}</p>
+                    <p className="text-sm text-secondary-800 truncate">{decodeMimeHeader(att.filename)}</p>
                     <p className="text-xs text-secondary-400">{formatBytes(att.size_bytes)}</p>
                   </div>
                   <Button