Detect broken STT backends via real transcription probe in health check

_stt_backend_health now sends a minimal WAV to the transcription endpoint
after passing /health. A 500 response marks the backend unavailable,
catching containers that pass health checks but crash on model load (e.g.
CTranslate2 built without CUDA support).

Error messages from _transcribe_audio now include the backend URL and
replace generic 'Internal Server Error' with an actionable explanation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.py

@@ -1903,6 +1903,20 @@ def _stt_backend_api_key(settings: dict, backend: str) -> str:
     return settings.get("whisper_api_key", "").strip()
 
 
+def _make_minimal_wav() -> bytes:
+    """44-byte WAV header with 1 frame of silence — smallest valid WAV."""
+    import struct
+    num_frames = 1
+    data = b"\x00\x00"
+    header = struct.pack(
+        "<4sI4s4sIHHIIHH4sI",
+        b"RIFF", 36 + len(data), b"WAVE",
+        b"fmt ", 16, 1, 1, 16000, 32000, 2, 16,
+        b"data", len(data),
+    )
+    return header + data
+
+
 def _stt_backend_health(url: str) -> tuple[bool, list[str]]:
     base = _validate_http_url(url, allow_private=True).rstrip("/")
     models: list[str] = []
@@ -1925,6 +1939,30 @@ def _stt_backend_health(url: str) -> tuple[bool, list[str]]:
                     models.append(item)
     except Exception:
         pass
+    if not ok:
+        return False, models
+    # Verify the transcription endpoint actually works (catches broken builds like
+    # CTranslate2 containers compiled without CUDA that pass /health but fail on load)
+    try:
+        wav = _make_minimal_wav()
+        for path in ("/v1/audio/transcriptions", "/transcribe"):
+            try:
+                r = requests.post(
+                    f"{base}{path}",
+                    files={"file": ("probe.wav", wav, "audio/wav")},
+                    data={"model": "whisper-1", "response_format": "text"},
+                    timeout=8,
+                )
+                if r.status_code == 500:
+                    ok = False  # backend is broken (model load failed etc.)
+                break  # 200, 400, 404, 422 all mean the endpoint exists and model loaded
+            except requests.exceptions.ConnectionError:
+                ok = False
+                break
+            except Exception:
+                pass  # timeout or other — don't mark as broken, just skip
+    except Exception:
+        pass
     return ok, models
 
 
@@ -2039,7 +2077,9 @@ def _transcribe_audio(src: Path, settings: dict, backend: str = "configured") ->
             detail = body.get("detail") or body.get("error") or body.get("message") or str(body)
         except Exception:
             detail = resp.text[:300].strip()
-        raise RuntimeError(f"STT backend returned HTTP {resp.status_code}: {detail}")
+        if not detail or detail.lower() in {"internal server error", "unknown error"}:
+            detail = f"HTTP {resp.status_code} — backend may be misconfigured or missing CUDA support"
+        raise RuntimeError(f"STT ({stt_url}): {detail}")
     return _transcription_text_from_response(resp), backend