fix(worker): reject a download with 0-byte/missing tracks (#16 partial)

Completeness only compared the adapter's self-reported track_count to expected,
so a silently-skipped track (0-byte placeholder — see the Qobuz 24K Magic bug)
imported as a "complete" album. Now the verify also counts the NON-EMPTY audio
actually staged and fails "incomplete download" when fewer than expected are
present (guarded with `staged and ...` so fake adapters that stage nothing keep
using the count path). A real broken download now goes needs_attention (or falls
through to another source) instead of importing a dead track.

Full per-track duration matching against MB recording lengths (the rest of #16)
remains a larger follow-up. worker 224 tests / 7-skip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_incomplete_download.py

@@ -0,0 +1,39 @@
+"""A track that silently fails to download leaves a 0-byte placeholder but the adapter still
+reports the full count; the pipeline must reject it (incomplete) rather than import a broken
+album. (Motivated by Bruno Mars "24K Magic" track 1 downloading as 0 bytes.)"""
+import os
+
+from lyra_worker.adapters.fakes import FakeQobuz
+from lyra_worker.claim import claim_next
+from lyra_worker.pipeline import run_pipeline
+from lyra_worker.types import DownloadResult
+from tests.conftest import insert_request
+
+
+class _ZeroByteTrackFake(FakeQobuz):
+    """Stages (n-1) real tracks + one 0-byte placeholder but reports the full count n."""
+    def download(self, candidate, dest, on_progress):
+        os.makedirs(dest, exist_ok=True)
+        n = candidate.track_count
+        for i in range(n - 1):
+            with open(os.path.join(dest, f"{i + 1:02d} Track.flac"), "wb") as fh:
+                fh.write(b"audio")
+        with open(os.path.join(dest, f"{n:02d} Dead.flac"), "wb") as fh:
+            fh.write(b"")  # the silently-skipped track
+        return DownloadResult(ok=True, path=dest, track_count=n)
+
+
+def test_zero_byte_track_fails_completeness(conn):
+    job_id = insert_request(conn, artist="Radiohead", album="In Rainbows")
+    claim_next(conn)
+    run_pipeline(conn, job_id, [_ZeroByteTrackFake()], dest_root="/tmp/lib-zbt")
+
+    with conn.cursor() as cur:
+        cur.execute('SELECT state, error FROM "Job" WHERE id = %s', (job_id,))
+        state, error = cur.fetchone()
+    assert state == "needs_attention"
+    assert error == "incomplete download"
+    # nothing was imported
+    with conn.cursor() as cur:
+        cur.execute('SELECT count(*) FROM "LibraryItem" WHERE artist = %s', ("Radiohead",))
+        assert cur.fetchone()[0] == 0