fix(db): apply sqlc's actual output for candidate_artist_tags — #2376

Three divergences in the hand-written generated file, all caught by
verify-generate on the first run. Two are sqlc rules I had wrong:

1. When a query's SELECT list exactly matches a table's columns in order,
   sqlc REUSES the model struct rather than emitting a bespoke Row type.
   So ListCandidateArtistTagsForMbids returns []CandidateArtistTag, and
   ListCandidateArtistTagsForMbidsRow should never have existed.

2. models.go is ordered by GO STRUCT NAME, not table name. Table order
   would put candidate_artist_tag_state before candidate_artist_tags;
   sqlc emits CandidateArtistTag before CandidateArtistTagState. The
   earlier slice-3 observation ("ordered by table name") was consistent
   with both orderings and so never discriminated — this case does.

3. sqlc smart-quotes a doubled '' inside a promoted comment into a
   typographic ”. Reworded the prose to say "the empty string" instead of
   encoding a mangling into the source.

Note the integration lane PASSED on the broken push while this failed.
That is #2380's lesson landing again, and the reason the check exists:
valid SQL executing against real Postgres proves nothing about whether
the committed Go matches its source.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/db/queries/candidate_artist_tags.sql

@@ -20,10 +20,11 @@
 -- Already-in-library candidates are skipped: they have an artists row, so
 -- their tags belong in track_tags, and the suggestion query filters them out
 -- anyway. $1 = current tag_sources_version, $2 = limit.
--- candidate_name is coalesced to '' so it lands non-nullable in Go: the name is
--- only a Last.fm lookup key, and empty simply means "MBID-keyed providers only",
--- which the provider chain already handles. max() is an arbitrary-but-
--- deterministic pick when several seeds spell the same MBID differently.
+-- candidate_name is coalesced to the empty string so it lands non-nullable in
+-- Go: the name is only a Last.fm lookup key, and empty simply means "MBID-keyed
+-- providers only", which the provider chain already handles. max() is an
+-- arbitrary-but-deterministic pick when several seeds spell one MBID
+-- differently.
 SELECT u.candidate_mbid,
        coalesce(max(u.candidate_name), '')::text AS candidate_name,
        sum(u.score)::float8                      AS total_score