fix(audio): serve real Content-Type so imports decode in WaveSurfer

/audio/{id} and /audio/version/{id} hardcoded media_type="audio/wav" on
the FileResponse. That was a no-op when every generation came out of
TTS (everything on disk was a .wav anyway), but imported audio keeps
its source format — .mp3 / .m4a / .ogg — and the WaveSurfer MediaElement
backend uses an <audio> tag that checks Content-Type before letting the
clip play, so an MP3 announced as audio/wav silently failed to load.

Both endpoints now derive the type via mimetypes.guess_type and fall
back to audio/wav for unknown suffixes. Download filenames also keep
the real extension instead of always saying ".wav".

backend/routes/audio.py

@@ -1,5 +1,8 @@
 """Audio file serving endpoints."""
 
+import mimetypes
+from pathlib import Path
+
 from fastapi import APIRouter, Depends, HTTPException
 from fastapi.responses import FileResponse
 from sqlalchemy.orm import Session
@@ -11,6 +14,16 @@ from ..database import get_db
 router = APIRouter()
 
 
+def _audio_media_type(path: Path) -> str:
+    """Derive the Content-Type from the file extension.
+
+    Imported audio retains its source format (.mp3, .m4a, .ogg, …) so a
+    blanket ``audio/wav`` would mislead strict clients trying to decode
+    via the response header instead of sniffing the bytes."""
+    guessed, _ = mimetypes.guess_type(path.name)
+    return guessed or "audio/wav"
+
+
 @router.get("/audio/version/{version_id}")
 async def get_version_audio(version_id: str, db: Session = Depends(get_db)):
     """Serve audio for a specific version."""
@@ -26,8 +39,8 @@ async def get_version_audio(version_id: str, db: Session = Depends(get_db)):
 
     return FileResponse(
         audio_path,
-        media_type="audio/wav",
-        filename=f"generation_{version.generation_id}_{version.label}.wav",
+        media_type=_audio_media_type(audio_path),
+        filename=f"generation_{version.generation_id}_{version.label}{audio_path.suffix}",
     )
 
 
@@ -44,8 +57,8 @@ async def get_audio(generation_id: str, db: Session = Depends(get_db)):
 
     return FileResponse(
         audio_path,
-        media_type="audio/wav",
-        filename=f"generation_{generation_id}.wav",
+        media_type=_audio_media_type(audio_path),
+        filename=f"generation_{generation_id}{audio_path.suffix}",
     )