feat(extraction-service): add /api/transcribe route — speech-to-text via OpenAI Whisper API
- POST /api/transcribe: download audio from URL, call Whisper API, return transcript
- Types: TranscribeRequestSchema (Zod), TranscribeResponse, SUPPORTED_AUDIO_TYPES
- Guards: 25MB size limit, 30s download timeout, 120s Whisper timeout, 429 rate limit
- Config: OPENAI_API_KEY, OPENAI_BASE_URL, WHISPER_MODEL env vars
- 12 new tests (schema validation + constants)
- Registered in server.ts alongside extract + task routes
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