Transcription: config-driven OpenAI-compatible STT with OpenAI fallback (server-26#297, server-26#83)

STT_PROVIDER=auto picks Groq whisper-large-v3-turbo only when GROQ_API_KEY is
set (else OpenAI, unchanged); compat covers any OpenAI-compatible server via
STT_BASE_URL/STT_API_KEY. A failed primary falls back to OpenAI STT_MODEL on
the same downloaded audio; the fallback reports to its own ai_health tier so
a dead primary stays red. The non-OpenAI provider gets a token-budgeted
glossary prompt (area places, local_knowledge, vocabulary; never ten-codes),
guarded by a narrow prompt-echo check. A per-30s-window zlib compression
check (Whisper's 2.4) adds a noise gate that survives correction being off
(#83, partial). Call docs record stt_provider/model/audio_seconds/confidence.
The sweep hold-back counts the fallback attempt. gate_chatter added to
chatter_classifier (used by transcription to skip correction when
CHATTER_GATE_ENABLED). GROQ_API_KEY templated with default ''.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/config.py

@@ -28,8 +28,39 @@ class Settings(BaseSettings):
 
     # OpenAI (STT + intelligence)
     openai_api_key: Optional[str] = None
+    # The OpenAI transcription model. Used when OpenAI is the STT provider AND
+    # as the fallback when another provider fails (server-26#297).
     stt_model: str = "whisper-1"  # whisper-1 | gpt-4o-mini-transcribe | gpt-4o-transcribe
 
+    # ── STT provider (server-26#297) ──────────────────────────────────────────
+    # Any OpenAI-compatible /audio/transcriptions endpoint. Groq serves
+    # whisper-large-v3-turbo at $0.04/audio-hour against whisper-1's $0.36, same
+    # API (min billed length 10s per request, prompt capped at 224 tokens --
+    # console.groq.com/docs/speech-to-text, checked 2026-10-27).
+    #   auto   -- groq when GROQ_API_KEY is set, compat when STT_BASE_URL is
+    #             set, else openai. The default, so a deploy without either
+    #             behaves exactly as before.
+    #   openai -- OpenAI only (stt_model).
+    #   groq   -- Groq (stt_compat_model), OpenAI fallback.
+    #   compat -- any other OpenAI-compatible server: stt_base_url + stt_api_key
+    #             + stt_compat_model (e.g. a self-hosted faster-whisper, #296).
+    stt_provider: str = "auto"
+    stt_base_url: Optional[str] = None   # None -> the provider's own default
+    stt_api_key: Optional[str] = None    # None -> GROQ_API_KEY / OPENAI_API_KEY
+    groq_api_key: Optional[str] = None
+    stt_compat_model: str = "whisper-large-v3-turbo"  # model on a non-OpenAI provider
+    # Retry a failed non-OpenAI request once on OpenAI stt_model. Needs
+    # OPENAI_API_KEY; without it a primary failure just means no transcript.
+    stt_fallback_to_openai: bool = True
+    # Domain vocabulary in the STT prompt (area places, local_knowledge terms,
+    # vocabulary). Applied to the non-OpenAI provider only: whisper-1 echoed
+    # vocabulary into silence before (see transcription.py), so the OpenAI
+    # path -- including the fallback -- keeps today's fixed prompt.
+    stt_vocab_prompt_enabled: bool = True
+    # Whisper keeps only the LAST 224 prompt tokens and silently drops the
+    # rest. The prompt is built against this estimated budget, with headroom.
+    stt_prompt_max_tokens: int = 200
+
     # AI request timeouts (server-26#153). Without these the OpenAI SDK waits
     # up to 600s per attempt and retries twice, and the google-genai client
     # waits forever, so a hung provider call had no upper bound at all — and