Fix: torrent/usenet album bundle hard-fails on 'no results' instead of falling back

A torrent-first (or usenet-first) hybrid download would freeze at
"Torrent searching for release 0%" and never move to the next source when
Prowlarr returned no results for the album. Reported by Cezar:
  [Album Bundle] torrent flow failed for '...': No torrent results found

Cause: the album-bundle dispatch only returns to the per-track flow (which,
in hybrid mode, tries the next configured source) when the plugin's failure
outcome carries fallback=True; otherwise it marks the batch failed and stops.
Both plugins set fallback=True on their 'results found but none matched the
album' branch, but the adjacent 'no results at all' branch set only an error
and no fallback flag -- so zero results hard-failed while wrong results fell
back. Backwards, and soulseek's plugin already defaults fallback=True for
exactly this reason.

Fix: set fallback=True on the no-results branch in torrent.py and usenet.py.
The dispatch's fallback handling (return False -> per-track flow) was already
correct and is unchanged.

The only consumer of download_album_to_staging is the dispatch, which reads
the result via .get('fallback'), so the change is additive and locally
contained.

Tests: new test_torrent_album_to_staging_no_results_flags_fallback and
test_usenet_album_to_staging_no_results_flags_fallback assert the plugins now
emit fallback=True on an empty search; the existing torrent no-results test is
extended with the same assertion. Existing dispatch tests already pin
fallback=True -> per-track flow. Full downloads/plugins/adapters sweep: 690
passed.

core/download_plugins/torrent.py

@@ -494,7 +494,14 @@ class TorrentDownloadPlugin(DownloadSourcePlugin):
         candidates = [r for r in search_results
                       if r.protocol == 'torrent' and (r.magnet_uri or r.download_url)]
         if not candidates:
+            # Album isn't available on this source. Mark the failure as
+            # fallback-eligible so the dispatch returns to the per-track flow
+            # instead of hard-failing the batch — in hybrid mode that lets the
+            # next configured source take over. Without this flag a torrent-first
+            # hybrid would get stuck at "searching" forever when Prowlarr
+            # returns nothing, never trying the other sources.
             result['error'] = f'No torrent results found for "{query}"'
+            result['fallback'] = True
             return result
 
         picked = pick_best_album_release(

core/download_plugins/usenet.py

@@ -465,7 +465,12 @@ class UsenetDownloadPlugin(DownloadSourcePlugin):
         candidates = [r for r in search_results
                       if r.protocol == 'usenet' and r.download_url]
         if not candidates:
+            # Album isn't available on this source — fall back to the per-track
+            # flow (next configured source in hybrid mode) rather than hard-
+            # failing the whole batch. Mirrors the torrent plugin + soulseek's
+            # default fallback contract.
             result['error'] = f'No usenet results found for "{query}"'
+            result['fallback'] = True
             return result
 
         picked = pick_best_album_release(

tests/test_torrent_usenet_plugins.py

@@ -589,9 +589,45 @@ def test_torrent_album_to_staging_ignores_candidates_without_download_url(tmp_pa
 
     assert outcome['success'] is False
     assert 'No torrent results' in outcome['error']
+    # Regression (Cezar): "no results" must be fallback-eligible so a
+    # torrent-first hybrid returns to the per-track flow (next source)
+    # instead of the dispatch marking the batch failed and freezing at
+    # "Torrent searching for release 0%".
+    assert outcome.get('fallback') is True
     fake_adapter.add_torrent.assert_not_called()
 
 
+def test_torrent_album_to_staging_no_results_flags_fallback(tmp_path: Path) -> None:
+    """Empty Prowlarr search → fallback-eligible failure, not terminal."""
+    plugin = TorrentDownloadPlugin()
+    fake_adapter = MagicMock()
+    fake_adapter.is_configured.return_value = True
+    with patch.object(plugin, 'is_configured', return_value=True), \
+         patch.object(plugin._prowlarr, 'search', new=AsyncMock(return_value=[])), \
+         patch('core.download_plugins.torrent.get_active_torrent_adapter', return_value=fake_adapter):
+        outcome = plugin.download_album_to_staging('GNX', 'Kendrick Lamar', str(tmp_path))
+    assert outcome['success'] is False
+    assert 'No torrent results' in outcome['error']
+    assert outcome.get('fallback') is True
+    fake_adapter.add_torrent.assert_not_called()
+
+
+def test_usenet_album_to_staging_no_results_flags_fallback(tmp_path: Path) -> None:
+    """Same contract for usenet: an empty search must fall back to the
+    per-track flow rather than hard-failing the album batch."""
+    plugin = UsenetDownloadPlugin()
+    fake_adapter = MagicMock()
+    fake_adapter.is_configured.return_value = True
+    with patch.object(plugin, 'is_configured', return_value=True), \
+         patch.object(plugin._prowlarr, 'search', new=AsyncMock(return_value=[])), \
+         patch('core.download_plugins.usenet.get_active_usenet_adapter', return_value=fake_adapter):
+        outcome = plugin.download_album_to_staging('GNX', 'Kendrick Lamar', str(tmp_path))
+    assert outcome['success'] is False
+    assert 'No usenet results' in outcome['error']
+    assert outcome.get('fallback') is True
+    fake_adapter.add_nzb.assert_not_called()
+
+
 def test_registry_includes_torrent_and_usenet() -> None:
     """The registry decides what shows up in the orchestrator's
     iteration helpers. If we forget to register a new plugin the