fix(backend): convert unsupported formats to WAV before SaluteSpeech upload

MP4/webm/m4a files are now auto-converted to WAV (PCM_S16LE) via ffmpeg
before uploading to SaluteSpeech API. Follows the same pattern as Google
Speech's _convert_local_to_ogg.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cpv3/modules/transcription/service.py

@@ -671,6 +671,33 @@ def _build_document_from_salute_result(
     return builder.process_document(document)
 
 
+def _convert_to_wav_sync(input_path: str, sample_rate: int = 16000) -> tuple[str, Callable[[], None]]:
+    """Convert any audio/video to WAV (PCM signed 16-bit LE) using ffmpeg. Sync version."""
+    import os
+    import subprocess
+
+    with NamedTemporaryFile(suffix=".wav", delete=False) as out:
+        out_path = out.name
+
+    result = subprocess.run(
+        [
+            "ffmpeg", "-y", "-i", input_path,
+            "-vn", "-ac", "1", "-ar", str(sample_rate),
+            "-acodec", "pcm_s16le",
+            out_path,
+        ],
+        capture_output=True,
+    )
+    if result.returncode != 0:
+        raise RuntimeError(f"ffmpeg failed: {result.stderr.decode(errors='ignore')}")
+
+    def _cleanup() -> None:
+        if os.path.exists(out_path):
+            os.remove(out_path)
+
+    return out_path, _cleanup
+
+
 def _salute_transcribe_sync(
     *,
     local_file_path: str,
@@ -686,11 +713,18 @@ def _salute_transcribe_sync(
     ext = Path(local_file_path).suffix.lower()
     audio_encoding = SALUTE_ENCODING_MAP.get(ext)
     content_type = SALUTE_CONTENT_TYPE_MAP.get(ext)
+
+    # Convert unsupported formats (mp4, webm, m4a, etc.) to WAV via ffmpeg
+    cleanup_fn: Callable[[], None] | None = None
     if not audio_encoding or not content_type:
-        raise ValueError(ERROR_SALUTE_UNSUPPORTED_FORMAT.format(ext=ext))
+        wav_path, cleanup_fn = _convert_to_wav_sync(local_file_path, sample_rate)
+        local_file_path = wav_path
+        audio_encoding = "PCM_S16LE"
+        content_type = "audio/wav"
 
     salute_language = SALUTE_LANGUAGE_MAP.get(language or "", "ru-RU")
 
+    try:
         verify = str(settings.salute_ca_cert_path) if settings.salute_ca_cert_path else True
         with httpx.Client(verify=verify, timeout=30.0) as client:
             token = _get_salute_access_token(client)
@@ -714,6 +748,9 @@ def _salute_transcribe_sync(
             raw_result = _download_salute_result(client, token, response_file_id)
 
         return _build_document_from_salute_result(raw_result, language=salute_language)
+    finally:
+        if cleanup_fn is not None:
+            cleanup_fn()
 
 
 async def transcribe_with_salute_speech(