Fix API 500 errors when frontend sends display names instead of IDs. Use when:
(1) Backend API returns 500 Internal Server Error, (2) You're sending
speaker_name/user_name/display_name but backend expects file path or ID,
(3) Backend constructs file paths like os.path.join(DIR, speaker_name),
(4) Error mentions "file not found" or "invalid speaker". Common pattern in
file upload, TTS, and media processing apps.