BUG 1: Azure locale derivation produced 'en-EN' (invalid) for 2-letter codes.
→ Added toAzureLocale() with 28-language mapping table (en→en-US, pt→pt-BR, etc.)
→ Exported for testing; falls back to code-CODE for unmapped languages.
BUG 2: model field from request schema was silently dropped after provider refactor.
→ Added optional model field to TranscriptionInput interface.
→ OpenAI provider now uses input.model override (falls back to config.model).
→ Route passes model through to provider.transcribe().
GAP 4: SUPPORTED_AUDIO_TYPES was defined but never validated against.
→ Route now rejects unsupported content-types with a clear error message.
→ Allows application/octet-stream (Azure Blob SAS URLs often return this).
GAP 5: Client JSDoc still said 'via OpenAI Whisper API' — now 'via configured STT provider'.
GAP 8: Azure WAV content-type hardcoded samplerate=16000 — now generic audio/wav.
Tests: 42 transcription tests (was 35), 178 total passing.
→ toAzureLocale: 4 tests (locale mapping, passthrough, fallback, case-insensitive)
→ setSTT: 1 test (singleton override)
→ model passthrough: 2 tests (mock ignores, input accepts)
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