Add voice-to-text transcription for Telegram voice/audio messages using
any Whisper-compatible API (Groq by default, configurable endpoint).
- New TranscriptionConfig in config schema (enabled, api_url, model,
language, max_duration_secs) with serde defaults
- New transcription module: MIME detection, .oga→.ogg normalization,
size/format validation, Whisper API client
- Telegram: voice download pipeline (getFile → CDN download → transcribe),
listen loop fallback for voice messages, [Voice] prefix on transcribed text
- Proxy support via "transcription.groq" service key
- 18 new tests (MIME mapping, normalization, config roundtrip, voice
metadata parsing, builder wiring, format/size rejection)
Disabled by default (enabled: false). Fail-fast validation order:
size → format → API key.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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