fix: look tags up by artist name rather than by MusicBrainz id

Tag lookups asked by MusicBrainz id whenever Lidarr had one, which is always.
The reasoning was that an id cannot be ambiguous the way a name can. In practice
Last.fm's mbid index is stale and partial, and it answered "the artist you
supplied could not be found" for Devo, Escape the Fate, Blasterjaxx, Frank
Carter & the Rattlesnakes and a long tail of others -- artists whose Last.fm
pages plainly exist and carry precisely the tags the moods are built from. Devo
is tagged new wave, post-punk and 80s; Escape the Fate is tagged post-hardcore,
screamo and emocore. Both were skipped.

Ask by name first, which is the index Last.fm's own site runs on, and keep the
id only as a fallback for a name Lidarr spells differently.

Failures were also being dropped without recording the attempt, so every one of
those artists was re-queried on every subsequent pass, indefinitely. They are now
distinguished: an artist that neither key resolves is recorded as fetched with
no tags and not asked about again, while a genuine failure -- a rate limit, a
bad key -- is deliberately left unrecorded so the next pass retries it. Telling
the two apart needed the service's own error number, so LastfmError now carries
it.

tests/test_music_curator.py

@@ -1050,8 +1050,8 @@ def test_a_vibe_selects_by_tag(tmp_path):
     store, source, mirror = tagged_store(
         tmp_path,
         {
-            "artist-mbid-1": [{"name": "screamo", "count": 100}],
-            "artist-mbid-2": [{"name": "classic rock", "count": 100}],
+            "Played Band": [{"name": "screamo", "count": 100}],
+            "Silent Band": [{"name": "classic rock", "count": 100}],
         },
     )
     vibes = [{"name": "screamo", "tags": ["screamo"]}]
@@ -1066,7 +1066,7 @@ def test_a_vibe_selects_by_tag(tmp_path):
 def test_a_weakly_tagged_artist_is_below_the_threshold(tmp_path):
     """A single low-weight tag is not a genre, it is somebody's stray opinion."""
     store, source, mirror = tagged_store(
-        tmp_path, {"artist-mbid-1": [{"name": "screamo", "count": 3}]}
+        tmp_path, {"Played Band": [{"name": "screamo", "count": 3}]}
     )
     vibes = [{"name": "screamo", "tags": ["screamo"]}]
 
@@ -1079,7 +1079,7 @@ def test_a_vibe_can_be_restricted_by_release_year(tmp_path):
     """What separates eighties synth records from everything else a synthpop
     tag drags in."""
     store, source, mirror = tagged_store(
-        tmp_path, {"artist-mbid-1": [{"name": "synthpop", "count": 100}]}
+        tmp_path, {"Played Band": [{"name": "synthpop", "count": 100}]}
     )
     inside = [{"name": "eighties", "tags": ["synthpop"], "years": [1975, 1992]}]
     outside = [{"name": "nineties", "tags": ["synthpop"], "years": [1993, 1999]}]
@@ -1127,3 +1127,74 @@ def test_a_bad_vibes_file_is_refused_up_front(tmp_path, content):
 
     with pytest.raises(ValueError):
         music_curator.load_vibes(str(path))
+
+
+def test_tags_are_looked_up_by_name_not_by_mbid(tmp_path):
+    """Last.fm's mbid index is stale: it cannot find Devo or Escape the Fate by
+    one, though their pages plainly exist. Its name index can."""
+    api, source, mirror = playlist_library(tmp_path)
+    store = store_at(tmp_path)
+    music_curator.index_library(
+        music_curator.Lidarr("http://lidarr", "key", transport=api), store
+    )
+    lastfm = FakeLastfm(tags={"Played Band": [{"name": "screamo", "count": 90}]})
+
+    music_curator.sync_tags(client_for(lastfm), store, NOW)
+
+    asked = [call for call in lastfm.calls if call.get("method") == "artist.getTopTags"]
+    # The name is always tried first; the mbid only appears as a fallback for
+    # the artist that the name could not resolve.
+    assert "artist" in asked[0]
+    assert [call for call in asked if call.get("artist") == "Played Band"]
+    assert not [call for call in asked if call.get("mbid") == "artist-mbid-1"]
+    assert store.scalar("SELECT COUNT(*) FROM artist_tag WHERE tag = 'screamo'") == 1
+
+
+def test_the_mbid_is_tried_when_the_name_is_not_found(tmp_path):
+    """Kept only for a name Lidarr spells differently to Last.fm."""
+    api, source, mirror = playlist_library(tmp_path)
+    store = store_at(tmp_path)
+    music_curator.index_library(
+        music_curator.Lidarr("http://lidarr", "key", transport=api), store
+    )
+    lastfm = FakeLastfm(tags={"artist-mbid-1": [{"name": "dnb", "count": 80}]})
+
+    music_curator.sync_tags(client_for(lastfm), store, NOW)
+
+    assert store.scalar("SELECT COUNT(*) FROM artist_tag WHERE tag = 'dnb'") == 1
+    asked = [call for call in lastfm.calls if call.get("method") == "artist.getTopTags"]
+    assert any("mbid" in call for call in asked)
+
+
+def test_an_artist_neither_key_resolves_is_recorded_and_not_retried(tmp_path):
+    """Otherwise every pass spends a request on it again, for ever."""
+    api, source, mirror = playlist_library(tmp_path)
+    store = store_at(tmp_path)
+    music_curator.index_library(
+        music_curator.Lidarr("http://lidarr", "key", transport=api), store
+    )
+    lastfm = FakeLastfm(tags={})
+
+    assert music_curator.sync_tags(client_for(lastfm), store, NOW) == 2
+    spent = len(lastfm.calls)
+
+    assert music_curator.sync_tags(client_for(lastfm), store, NOW) == 0
+    assert len(lastfm.calls) == spent
+
+
+def test_a_real_failure_is_not_recorded_so_the_next_pass_retries(tmp_path):
+    """A rate limit is not the same as an artist not existing."""
+    api, source, mirror = playlist_library(tmp_path)
+    store = store_at(tmp_path)
+    music_curator.index_library(
+        music_curator.Lidarr("http://lidarr", "key", transport=api), store
+    )
+    lastfm = FakeLastfm(
+        tags={"Played Band": [], "Silent Band": []},
+        outcomes=[{"error": 10, "message": "Invalid API key"}],
+    )
+
+    music_curator.sync_tags(client_for(lastfm), store, NOW)
+
+    # One artist failed hard and must still be pending.
+    assert store.scalar("SELECT COUNT(*) FROM artist_tag_fetched") == 1