fix(worker): cancel abandoned slskd transfers + cap download fall-through

Two compounding causes of 'many downloads of one album from different sources' in
slskd: (1) the slskd client raised on timeout/error WITHOUT cancelling the transfer
it enqueued, so as the pipeline fell through to other peers each left a transfer
running; (2) the incomplete-download fall-through was uncapped, grinding through
every candidate of a popular album (~200 Soulseek peers), amplified now that Qobuz
pacing pushes most jobs to Soulseek.

- SlskdClient.download now cancels its enqueued transfers (DELETE) on any abandon.
- The download loop caps at _MAX_DOWNLOAD_ATTEMPTS (6) candidates.

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

worker/tests/test_incomplete_fallthrough.py

@@ -89,3 +89,37 @@ def test_all_sources_incomplete_still_needs_attention(conn, tmp_path):
     run_pipeline(conn, job_id, [_Qobuz(deliver=2), _Soulseek(deliver=1)], dest_root=str(tmp_path))
     assert _state_error(conn, job_id) == ("needs_attention", "incomplete download")
     assert not (tmp_path / "Radiohead" / "In Rainbows").exists()  # nothing imported
+
+
+class _CountingIncompleteQobuz:
+    """Offers `n` candidates that all download but come back incomplete; counts download calls."""
+    name = "qobuz"
+    tier = 0
+
+    def __init__(self, n):
+        self.n = n
+        self.calls = 0
+
+    def health(self):
+        return True
+
+    def search(self, target):
+        q = _HIRES
+        return [Candidate(source="qobuz", source_ref=f"r{i}", matched_artist=target.artist,
+                          matched_album=target.album, quality=q, track_count=3, source_tier=0)
+                for i in range(self.n)]
+
+    def download(self, candidate, dest, on_progress):
+        self.calls += 1
+        _stage(dest, 1)  # 1 of 3 → always incomplete
+        return DownloadResult(ok=True, path=dest, track_count=1)
+
+
+def test_fall_through_is_capped(conn):
+    from lyra_worker.pipeline import _MAX_DOWNLOAD_ATTEMPTS
+    a = _CountingIncompleteQobuz(n=20)
+    job_id = insert_request(conn, artist="Radiohead", album="In Rainbows")
+    claim_next(conn)
+    run_pipeline(conn, job_id, [a], dest_root="/tmp/lib-cap")
+    assert _state_error(conn, job_id) == ("needs_attention", "incomplete download")
+    assert a.calls == _MAX_DOWNLOAD_ATTEMPTS  # capped, not all 20 peers tried