Serve audio as whatever it actually is

audio/mpeg was hardcoded at both points call audio is written and served, from
back when the node produced nothing but 16 kbps MP3. It now uploads FLAC, and a
browser will not play a FLAC body labelled audio/mpeg.

storage.py grows one extension -> Content-Type map, used by the GCS upload and
by /media. Keyed off the object's real extension, so every existing .mp3
recording keeps working with no migration -- and _safe_audio_filename already
accepted .flac, so object naming needed nothing.

Also flags what this costs: /media sends the whole body with Accept-Ranges:
none, which was fine at ~60 KB per call and is not fine at ~1.3 MB/min. Noted
at the header and in DEFERRED.md, whose stated reason for deferring Range
support was the old file size.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

drb-c2-core/app/internal/storage.py

@@ -44,11 +44,33 @@ def _safe_audio_filename(filename: str, call_id: str) -> str:
     The original extension is preserved only if it's a known audio type.
     """
     ext = os.path.splitext(filename)[-1].lower() if filename else ""
-    if ext not in (".mp3", ".wav", ".ogg", ".m4a", ".aac", ".flac"):
+    if ext not in AUDIO_CONTENT_TYPES:
         ext = ".mp3"
     return f"{call_id}{ext}"
 
 
+# Extension → Content-Type. The node used to send nothing but 16 kbps MP3, so
+# "audio/mpeg" was hardcoded at every point audio is written or served; it now
+# sends FLAC (lossless, for Whisper's benefit — see call_recorder.py's AUDIO_*
+# constants) and a stored object mislabelled audio/mpeg will not play in a
+# browser. Old .mp3 objects keep working: the map is keyed off the real
+# extension, not off what the current node happens to produce.
+AUDIO_CONTENT_TYPES = {
+    ".flac": "audio/flac",
+    ".mp3":  "audio/mpeg",
+    ".wav":  "audio/wav",
+    ".ogg":  "audio/ogg",
+    ".m4a":  "audio/mp4",
+    ".aac":  "audio/aac",
+}
+
+
+def content_type_for(name: str) -> str:
+    """Content-Type for a stored audio object, by extension. Defaults to MP3."""
+    ext = os.path.splitext(name or "")[-1].lower()
+    return AUDIO_CONTENT_TYPES.get(ext, "audio/mpeg")
+
+
 async def upload_audio(data: bytes, filename: str, call_id: str = "") -> Optional[str]:
     """Upload audio bytes to GCS and return the canonical gs:// URI, or None if disabled."""
     if not settings.gcs_bucket:
@@ -65,7 +87,7 @@ async def upload_audio(data: bytes, filename: str, call_id: str = "") -> Optiona
         else:
             client = storage.Client()
         blob = client.bucket(settings.gcs_bucket).blob(blob_path)
-        blob.upload_from_string(data, content_type="audio/mpeg")
+        blob.upload_from_string(data, content_type=content_type_for(safe_name))
 
     try:
         await asyncio.to_thread(_upload)