feat: download ETA on the press bar from measured Soulseek throughput

Show a live "~Xm left" countdown on the download progress bar. The slskd download
loop already polls byte transfer every 3s; measure the actual throughput (EWMA-
smoothed bytes/sec), compute seconds-remaining from the album's total bytes, and
surface it. New nullable Job.downloadEtaSeconds (migration; web entrypoint runs
prisma migrate deploy), threaded through the on_progress callback (optional 2nd arg,
so other adapters/callers are unaffected — they report no ETA). API exposes it;
queue.tsx renders etaLabel() after the track count. Null when unknown (no sample yet
or a source that doesn't report bytes). Worker 330 tests, web 214 tests, both green;
verified live-rendered as "42% · 6/14 tracks · ~4m left".

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

worker/tests/test_slskd_cancel.py

@@ -6,7 +6,7 @@ import json
 import pytest
 
 import lyra_worker.adapters._slskd as slskd_mod
-from lyra_worker.adapters._slskd import SlskdClient, _parse_search_responses
+from lyra_worker.adapters._slskd import SlskdClient, _eta_seconds, _parse_search_responses
 
 
 class _Resp:
@@ -63,7 +63,7 @@ def test_stalled_peer_is_abandoned_and_cancelled(tmp_path, monkeypatch):
     fake = slskd_mod.requests
 
     with pytest.raises(TimeoutError):
-        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda _p: None)
+        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda *_a: None)
     assert any(u.endswith("/api/v0/transfers/downloads/peerA/t1") for u in fake.deleted)
 
 
@@ -72,7 +72,7 @@ def test_error_state_is_cancelled(tmp_path, monkeypatch):
     monkeypatch.setattr(slskd_mod.time, "sleep", lambda _s: None)
     fake = slskd_mod.requests
     with pytest.raises(RuntimeError):
-        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda _p: None)
+        _client(tmp_path).download(_REF, str(tmp_path / "dest"), lambda *_a: None)
     assert any(u.endswith("/peerA/t1") for u in fake.deleted)
 
 
@@ -92,7 +92,7 @@ def test_slow_but_progressing_peer_is_not_abandoned(tmp_path, monkeypatch):
     monkeypatch.setattr(slskd_mod.time, "sleep", lambda _s: None)
     monkeypatch.setattr(slskd_mod, "_STALL_POLLS", 2)  # would fire early if progress were ignored
 
-    out = _client(tmp_path).download(ref, str(tmp_path / "dest"), lambda _p: None)
+    out = _client(tmp_path).download(ref, str(tmp_path / "dest"), lambda *_a: None)
     assert out["track_count"] == 1
     assert slskd_mod.requests.deleted == []  # completed cleanly → nothing cancelled
 
@@ -108,3 +108,39 @@ def test_parse_ranks_free_and_fast_peers_first():
     ]
     peers = [json.loads(c["source_ref"])["username"] for c in _parse_search_responses(responses)]
     assert peers == ["fast", "slow", "queued"]  # free+fast first; no-slot peer last despite speed
+
+
+def test_eta_seconds_from_measured_rate():
+    assert _eta_seconds(1000, 250, 250.0) == 3  # 750 bytes left / 250 B/s
+    assert _eta_seconds(1000, 0, 100.0) == 10
+
+
+def test_eta_seconds_unknown_cases():
+    assert _eta_seconds(1000, 250, 0.0) is None      # no speed sample yet
+    assert _eta_seconds(0, 0, 100.0) is None          # unknown total size
+    assert _eta_seconds(1000, 1000, 100.0) is None    # already complete
+
+
+def test_download_reports_shrinking_eta(tmp_path, monkeypatch):
+    # A 900-byte file arriving 300 bytes per 3s poll → measured speed 100 B/s → ETA counts down
+    # (6s, 3s) then None on completion. on_progress gets (fraction, eta_seconds).
+    (tmp_path / "01.flac").write_bytes(b"x" * 900)  # so _retrieve finds the finished file
+    ref = json.dumps({"username": "peerA", "files": [{"filename": r"d\Album\01.flac", "size": 900}]})
+    polls = [
+        {"directories": [{"files": [{"filename": r"d\Album\01.flac", "size": 900,
+                                     "state": "InProgress", "bytesTransferred": b, "id": "t1"}]}]}
+        for b in (300, 600)
+    ] + [
+        {"directories": [{"files": [{"filename": r"d\Album\01.flac", "size": 900,
+                                     "state": "Completed, Succeeded", "bytesTransferred": 900, "id": "t1"}]}]}
+    ]
+    monkeypatch.setattr(slskd_mod, "requests", _SeqRequests(polls))
+    monkeypatch.setattr(slskd_mod.time, "sleep", lambda _s: None)
+    clock = iter(float(i) * 3 for i in range(100))  # 0,3,6,… deterministic monotonic time
+    monkeypatch.setattr(slskd_mod.time, "monotonic", lambda: next(clock))
+
+    calls: list = []
+    _client(tmp_path).download(ref, str(tmp_path / "dest"), lambda *a: calls.append(a))
+
+    etas = [a[1] for a in calls if len(a) > 1]
+    assert etas == [6, 3, None]  # counts down at 100 B/s, then None once complete