Move build_narrator_roster, build_speaker_roster, match_configured_speaker,
apply_speaker_config_to_roster, prepare_speaker_metadata from
webui/routes/utils/voice.py to domain/speaker_metadata.py.
- prepare_speaker_metadata now accepts optional inject_recommended callback
for UI-specific voice enrichment
- 47 new tests in test_domain_speaker_metadata.py
- All 1509 tests pass
