fix(worker): raise http.client header limit so Qobuz tracks stop failing

Bruno Mars "24K Magic" track 1 repeatedly downloaded as a 0-byte file. The
worker log showed the real cause: streamrip's track download hit
HTTPException("got more than 100 headers") — Qobuz's CDN returns >100 response
headers for some tracks and Python's http.client rejects them at the default
_MAXHEADERS=100, so streamrip skips the track and leaves an empty placeholder.

- Raise http.client._MAXHEADERS to 1000 on import of the Qobuz adapter, so those
  responses parse and the track downloads.
- _count_staged_audio now skips 0-byte files (a failed-track placeholder is not a
  real track) — improves the download-progress estimate and stops counting dead
  tracks. (A verify-level "reject 0-byte tracks" safety net is a follow-up under
  #16 per-track duration matching — it needs the pipeline fakes to write real
  files, deferred to avoid disproportionate churn here.)

worker 223 tests / 7-skip.

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

worker/tests/test_download_progress.py

@@ -41,14 +41,21 @@ def test_on_progress_clamps_out_of_range(conn):
 def test_count_staged_audio_counts_nested_audio_files_only(tmp_path):
     nested = tmp_path / "Artist" / "Album"
     nested.mkdir(parents=True)
-    (nested / "01.flac").write_bytes(b"")
-    (nested / "02.flac").write_bytes(b"")
-    (nested / "03.flac").write_bytes(b"")
-    (nested / "cover.jpg").write_bytes(b"")
+    (nested / "01.flac").write_bytes(b"audio")
+    (nested / "02.flac").write_bytes(b"audio")
+    (nested / "03.flac").write_bytes(b"audio")
+    (nested / "cover.jpg").write_bytes(b"img")
 
     assert _count_staged_audio(str(tmp_path)) == 3
 
 
+def test_count_staged_audio_skips_zero_byte_files(tmp_path):
+    # a 0-byte file is a failed-track placeholder (e.g. streamrip skipping a track) → not a track
+    (tmp_path / "01 Good.flac").write_bytes(b"audio")
+    (tmp_path / "02 Dead.flac").write_bytes(b"")
+    assert _count_staged_audio(str(tmp_path)) == 1
+
+
 def test_count_staged_audio_empty_dir_is_zero(tmp_path):
     assert _count_staged_audio(str(tmp_path)) == 0