gateway: add tinfoil/voxtral-small-24b, Tinfoil's token-priced STT model

Tinfoil's catalog prices exactly one audio model per token: voxtral-small-24b
($0.20 in / $0.60 out per 1M, $0 per request). Wiring it gives members an
STT route without the whisper $0.01/request floor (co-op/support#48).

Two billing shapes on one model entry:
- chat route (input_audio): returns real token usage -> bills exactly per token.
- /v1/audio/transcriptions: Tinfoil returns NO token counts (duration only),
  so per-token billing cannot fire; Tinfoil also tokenizes audio in ~30s
  chunks (~375-380 tokens each regardless of clip length), making the
  chunk-equivalent ~$0.00008/request. Declared input_cost_per_request with
  that measured value, added by patch_audio_request_cost.py.

patch_audio_request_cost.py: scope the per-request charge to the audio call
types (transcription/atranscription/speech/aspeech) so this model's chat
calls bill per token and are not double-billed; extend the diagnostic
name-gate to 'voxtral'.

Measured on the live catalog + sidecar 2026-11-06 (see issue #48 follow-up).

patch_audio_request_cost.py

@@ -27,7 +27,13 @@ from the per-deployment ``custom_pricing`` the logging layer builds out of our
 call metadata.
 
 Scope: nothing changes unless a model declares ``input_cost_per_request``, so
-this is inert for every other model on the gateway.
+this is inert for every other model on the gateway. Since 2026-11-06 the charge
+is added only on the AUDIO call types (transcription/atranscription/speech/
+aspeech): a model that also carries per-token pricing (``voxtral-small-24b``)
+bills its chat calls exactly per token, and a per-request charge on top of
+that would double-bill them. The audio endpoints are the paths that cannot
+price per token (the provider returns no token counts there), so the flat
+charge belongs to them alone.
 
 Upstream: no issue filed; LiteLLM has no per-request audio pricing as of 1.103.2.
 """
@@ -58,7 +64,9 @@ HELPER = '''def _coop_per_request_cost(
     """
     def _is_audio(name: Any) -> bool:
         text = str(name or "").lower()
-        return "tts" in text or "whisper" in text
+        return (
+            "tts" in text or "whisper" in text or "voxtral" in text
+        )
 
     cost_map = {}
     try:
@@ -140,17 +148,34 @@ PATCHED_RETURN = """                region_name=region_name,
         # per-request charge for models LiteLLM can only price per unit
         # (Tinfoil's voxtral-tts, whisper-large-v3-turbo). Inert when the model
         # declares nothing; the early "provider reported the cost" return above
-        # is unaffected.
+        # is unaffected. Scoped to the AUDIO call types (2026-11-06): a model
+        # that ALSO carries per-token pricing (voxtral-small-24b) bills its
+        # chat calls exactly per token and must not take the per-request
+        # charge on top of that.
         try:
-            _coop_pr = _coop_per_request_cost(
-                model=model,
-                litellm_model_name=litellm_model_name,
-                custom_pricing=custom_pricing,
-                litellm_logging_obj=litellm_logging_obj,
+            _coop_audio_call = call_type in (
+                "transcription",
+                "atranscription",
+                "speech",
+                "aspeech",
+            )
+            _coop_pr = (
+                _coop_per_request_cost(
+                    model=model,
+                    litellm_model_name=litellm_model_name,
+                    custom_pricing=custom_pricing,
+                    litellm_logging_obj=litellm_logging_obj,
+                )
+                if _coop_audio_call
+                else 0.0
             )
             response_cost = (response_cost or 0.0) + _coop_pr
             _coop_name = str(model or "") + str(litellm_model_name or "")
-            if "tts" in _coop_name.lower() or "whisper" in _coop_name.lower():
+            if _coop_audio_call and (
+                "tts" in _coop_name.lower()
+                or "whisper" in _coop_name.lower()
+                or "voxtral" in _coop_name.lower()
+            ):
                 print(
                     f"[coop-audio] add per_request={_coop_pr!r} -> response_cost={response_cost!r}",
                     flush=True,