fix: add error handling for podcast generation failures (#1257)

* fix: add error handling for podcast generation failures

When TTS processing fails, the system was generating 0-second audio files
without any error indication. This fix adds:

1. Track failed TTS lines and log warning with indices
2. Raise ValueError when all TTS generation fails with helpful message
3. Check for empty audio output in mix_audio and raise error
4. Log success/failure ratio for debugging

Fixes #30

* fix: address Copilot review feedback

- Use `not audio` to catch both None and empty bytes
- Log failed lines with 1-based indices for user-friendly output
- Handle empty script case with clear error message
- Validate env vars before ThreadPoolExecutor for fast-fail on config errors

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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>