feat(autotag): answer whether an album has a confident match

`MatchForAlbum(albumID)` is the question the album detail page needs to
ask on open: does the autotagger already have something confident to
say about this album, and what would applying it do.

**It costs no MusicBrainz request.** Everything it needs is on disk —
`tagging_items` carries the top score and release from the background
prefetch, `tagging_candidates` durably holds the scored list. The rate
limiters here are shared with every page the user can open, so a lookup
that fires on page load must not join that queue; a folder nobody has
scored yet answers "nothing", rather than scoring it now.

**The tier is computed, not read.** `tagging_items.score` is the raw
number and `Recommend` is what turns it into a claim, capping it for an
ambiguous runner-up, an incomplete alignment or a folder too small to
corroborate itself. Filtering on the stored score would promise
confidence the scorer had explicitly withheld — which the two-track
test pins.

**Nothing is said about an album the user has already answered for.**
Only a `pending` group qualifies: `confirmed` covers both a finished
apply and an explicit "leave as is", and arguing with the second would
be actively wrong.

The join is `audio_files.group_key`, not a key derived from the folder
path, because a group carved out of a mixed-bag folder is keyed on its
tags — so a path-derived key would find nothing for exactly the
messiest libraries this helps. `GroupCount` is returned because a
multi-disc album is one group per disc: a caller that applied to "the
album" from a single button would retag one disc of three.

backend/database/sql/queries/tagging_items.sql

@@ -342,3 +342,35 @@ WHERE ti.status = 'pending'
   )
 ORDER BY ti.group_key
 LIMIT 1;
+
+-- name: GetTaggingItemsForAlbum :many
+-- Every tagging group holding a file of this album.
+--
+-- The join is `audio_files.group_key`, not a key derived from the
+-- album's folder path: a group carved out of a mixed-bag folder by
+-- SplitMixedFolder is keyed on its tags rather than on a directory,
+-- so a path-derived key finds nothing for exactly the messiest
+-- libraries this is meant to help.
+--
+-- Usually one row. A multi-disc album is one group per disc, which
+-- the caller has to know about rather than average over -- applying
+-- to "the album" would silently retag one disc of three.
+SELECT
+  ti.group_key,
+  ti.status,
+  ti.score,
+  ti.best_match_release_mbid,
+  ti.track_count,
+  ti.album_name,
+  ti.album_artist,
+  ti.synthetic
+FROM tagging_items ti
+WHERE ti.group_key IN (
+    SELECT DISTINCT af.group_key
+    FROM audio_files af
+    WHERE af.album_id = sqlc.arg(album_id) AND af.group_key != ''
+  )
+  AND ti.cleared_at IS NULL
+-- Best first, with an unscored group last rather than first: NULL
+-- sorts low in SQLite and DESC would put it at the top.
+ORDER BY ti.score IS NULL, ti.score DESC, ti.group_key;