feat: track download progress on Job (migration + worker poller + API)

Add Job.downloadProgress (0.0-1.0), derived from a worker background
poller that counts audio files landing in staging / expected track
count, since streamrip only reports 0/1. No UI yet (slice B).

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

worker/tests/test_download_progress.py

@@ -0,0 +1,22 @@
+import os
+
+from lyra_worker.pipeline import _count_staged_audio
+
+
+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"")
+
+    assert _count_staged_audio(str(tmp_path)) == 3
+
+
+def test_count_staged_audio_empty_dir_is_zero(tmp_path):
+    assert _count_staged_audio(str(tmp_path)) == 0
+
+
+def test_count_staged_audio_missing_dir_is_zero():
+    assert _count_staged_audio("/nonexistent/path/for/sure") == 0