fix(captures): clean up audio files when create_capture fails

The create flow wrote raw audio (and a transcoded .wav for non-wav
sources) to data/captures before the DB row was committed, so any
failure between the write and the commit — a webm that decoded to a
0-length array, a whisper model that errored mid-transcribe, a SQLite
contention on the commit — left the audio on disk with nothing pointing
at it. Over enough flaky uploads the directory grows without bound.

Now every path written before the commit is tracked in a list, and the
whole stretch from the first write to db.commit() runs inside a
try/except that unlinks each tracked file on raise and re-raises. The
transcode branch removes the raw file from the cleanup list only when
the unlink actually succeeds, so an OSError on the raw-path delete
still hands cleanup the original blob to retry.

backend/services/captures.py

@@ -71,7 +71,11 @@ async def create_capture(
         suffix = ".wav"
 
     raw_path = config.get_captures_dir() / f"{capture_id}{suffix}"
+    written_files: list[Path] = []
+
+    try:
         raw_path.write_bytes(audio_bytes)
+        written_files.append(raw_path)
 
         # Decode once with librosa — its audioread fallback handles webm/opus
         # via ffmpeg, which miniaudio (used inside mlx-audio's whisper) can't.
@@ -105,10 +109,13 @@ async def create_capture(
             # regardless of what format the client shipped.
             audio_path = config.get_captures_dir() / f"{capture_id}.wav"
             sf.write(str(audio_path), audio, sr, format="WAV")
+            written_files.append(audio_path)
             try:
                 raw_path.unlink()
             except OSError:
                 pass
+            else:
+                written_files.remove(raw_path)
 
         whisper = get_whisper_model()
         resolved_stt = stt_model or whisper.model_size
@@ -126,6 +133,16 @@ async def create_capture(
         db.add(row)
         db.commit()
         db.refresh(row)
+    except Exception:
+        # Anything between the first write and the commit means the audio on
+        # disk has no row pointing at it — clean up so data/captures doesn't
+        # accumulate orphan blobs across failed transcribes.
+        for path in written_files:
+            try:
+                path.unlink()
+            except OSError:
+                pass
+        raise
 
     return _to_response(row)