fix: replace assert with RuntimeError guard, truncate stderr before decode

- Replace assert in _worker with explicit RuntimeError guard (assert
strips in optimized mode, RuntimeError always fires)
- Truncate stderr to 2000 bytes before decoding in _run_demucs to
prevent unbounded error messages
- Add explanatory comment in test mock clarifying why htdemucs path
is used and that production code is model-agnostic via rglob()

processor.py

@@ -62,7 +62,7 @@ async def _run_demucs(job_id: str, input_path: str, output_path: str) -> None:
         _jobs[job_id].progress = 80
 
         if proc.returncode != 0:
-            raise RuntimeError(f"Demucs failed: {stderr.decode(errors='replace')}")
+            raise RuntimeError(f"Demucs failed: {stderr[:2000].decode(errors='replace')}")
 
         no_vocals = next(demucs_work_dir.rglob("no_vocals.mp3"), None)
         if no_vocals is None:
@@ -82,7 +82,8 @@ async def _run_demucs(job_id: str, input_path: str, output_path: str) -> None:
 
 async def _worker() -> None:
     """Background worker that pulls jobs from the queue and runs Demucs."""
-    assert _queue is not None, "Queue not initialized before worker started"
+    if _queue is None:
+        raise RuntimeError("Queue not initialized before worker started")
     while True:
         job_id, input_path, output_path = await _queue.get()
         try:

tests/test_processor.py

@@ -23,6 +23,8 @@ async def test_run_demucs_updates_state_on_success(tmp_path, monkeypatch):
         async def communicate(self):
             # Simulate Demucs writing output file
             demucs_work_dir = output_path.parent / f"demucs_{job_id}"
+            # "htdemucs" is the default Demucs model name; production code uses rglob()
+            # to be model-agnostic — this path just needs to be under demucs_work_dir
             out_file = demucs_work_dir / "htdemucs" / "input" / "no_vocals.mp3"
             out_file.parent.mkdir(parents=True, exist_ok=True)
             out_file.write_bytes(b"\xff\xfb" + b"\x00" * 50)