Add chunk-based Gemini transcription with VAD timestamp anchoring
Fix timestamp drift in Gemini transcription by sending audio as labeled
clips with explicit start times and durations. Gemini returns absolute
MM:SS timestamps which are parsed and mapped back to VAD segment boundaries.
Key changes:
- Send VAD segments as labeled clips: "Clip starting at MM:SS (Ns):"
- Gemini outputs absolute timestamps instead of clip references
- Parse MM:SS from output and clamp to valid segment range
- Skip audio reduction for Gemini (clips are already segmented)
- Improved prompt explains batch transcription with multiple speakers
This anchors timestamps to VAD boundaries rather than relying on Gemini's
internal clock, which drifts over long recordings.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>