fix(backend): return 404 instead of 500 for audio of failed generations (#893)

* fix(backend): return 404 instead of 500 for audio of failed generations

A failed generation stores an empty audio_path. resolve_storage_path("")
resolved to the data directory itself, which exists, so the route's 404
guard passed and FileResponse raised RuntimeError ("File at path .../data
is not a file"), surfacing as a 500.

- resolve_storage_path now returns None for empty paths
- audio routes check is_file() instead of exists() so directories never
  reach FileResponse
- GET /audio/{generation_id} reports "Generation failed; no audio
  available" when the generation status is failed

Co-Authored-By: Claude Fable 5 <[email protected]>

* fix(backend): reject empty Path objects in resolve_storage_path

Path("") is truthy, so the previous `if not path` guard only caught
None and empty strings. Callers such as database/migrations.py pass
Path objects, so an empty Path could still resolve to the data dir.
Check None separately and reject paths with no parts.

Also add regression tests asserting the version and sample audio
endpoints 404 when a stored path resolves to an existing directory
(guards the is_file() checks against regressing to exists()).

Addresses CodeRabbit review on PR #893.

Co-Authored-By: Claude Fable 5 <[email protected]>

* style(tests): drop parentheses on pytest.fixture decorator (ruff PT001)

Co-Authored-By: Claude Fable 5 <[email protected]>

* style(tests): satisfy Ruff naming rule

---------

Co-authored-by: Claude Fable 5 <[email protected]>
Co-authored-by: Jamie Pine <[email protected]>

backend/routes/audio.py

@@ -34,7 +34,7 @@ async def get_version_audio(version_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Version not found")
 
     audio_path = config.resolve_storage_path(version.audio_path)
-    if audio_path is None or not audio_path.exists():
+    if audio_path is None or not audio_path.is_file():
         raise HTTPException(status_code=404, detail="Audio file not found")
 
     return FileResponse(
@@ -52,8 +52,13 @@ async def get_audio(generation_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Generation not found")
 
     audio_path = config.resolve_storage_path(generation.audio_path)
-    if audio_path is None or not audio_path.exists():
-        raise HTTPException(status_code=404, detail="Audio file not found")
+    if audio_path is None or not audio_path.is_file():
+        detail = (
+            "Generation failed; no audio available"
+            if generation.status == "failed"
+            else "Audio file not found"
+        )
+        raise HTTPException(status_code=404, detail=detail)
 
     return FileResponse(
         audio_path,
@@ -72,7 +77,7 @@ async def get_sample_audio(sample_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Sample not found")
 
     audio_path = config.resolve_storage_path(sample.audio_path)
-    if audio_path is None or not audio_path.exists():
+    if audio_path is None or not audio_path.is_file():
         raise HTTPException(status_code=404, detail="Audio file not found")
 
     return FileResponse(