fix: force IPv4 in worker, count real Qobuz files, flatten nested output

- worker had no IPv6 route -> Qobuz CDN (dual-stack) failed ~half the tracks
  with 'Network is unreachable'; disable IPv6 in the container (IPv4 only).
- StreamripClient counted metadata tracks (always full) -> a partial download
  falsely imported; now count the actual files written (short count -> needs_attention).
- streamrip writes into a nested 'Artist - Album [..]/' folder the tagger never saw;
  flatten audio files up into the album dir so tagging/organization runs.

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

worker/tests/test_streamrip_flatten.py

@@ -0,0 +1,33 @@
+from lyra_worker.adapters._streamrip import _flatten_audio
+
+
+def test_flatten_moves_nested_audio_up_and_counts(tmp_path):
+    nested = tmp_path / "John Mayer - Continuum (2006) [FLAC] [24B-96kHz]"
+    nested.mkdir()
+    (nested / "01. Waiting.flac").write_bytes(b"x")
+    (nested / "02. Belief.flac").write_bytes(b"x")
+    (nested / "cover.jpg").write_bytes(b"x")  # non-audio stays behind
+
+    count = _flatten_audio(str(tmp_path))
+
+    assert count == 2
+    assert (tmp_path / "01. Waiting.flac").exists()
+    assert (tmp_path / "02. Belief.flac").exists()
+    assert not (nested / "01. Waiting.flac").exists()  # audio moved out of the nested folder
+
+
+def test_flatten_counts_flat_files_and_ignores_non_audio(tmp_path):
+    (tmp_path / "a.flac").write_bytes(b"x")
+    (tmp_path / "b.mp3").write_bytes(b"x")
+    (tmp_path / "notes.txt").write_bytes(b"x")
+    assert _flatten_audio(str(tmp_path)) == 2
+
+
+def test_flatten_partial_download_reports_true_short_count(tmp_path):
+    # streamrip continues past per-track failures; only 3 of an album's tracks landed.
+    nested = tmp_path / "sub"
+    nested.mkdir()
+    for n in ["01.flac", "02.flac", "03.flac"]:
+        (nested / n).write_bytes(b"x")
+    assert _flatten_audio(str(tmp_path)) == 3  # not the album's metadata count
+    assert not nested.exists()  # emptied subfolder removed