feat(database): shape the library like files, and shrink the catalog

Plans 013 and 014, the album page that prompted them, and the smaller
fixes they turned up. Changelog, largest first.

## The local library is shaped like files, not like MusicBrainz

`audio_files` carries its own tags and points at `albums` and
`artists`; `file_genres` is the one real many-to-many. `recordings`,
`release_group_recordings`, `artist_credit`, `artist_credit_artist`,
`recording_genres`, `release_groups` and `release_to_rg` are gone from
the local side, and with them a six-way join in every read, a
`MIN(release_group_id)` subquery in eleven queries and a
first-credited-artist subquery in nine. Measured on a real 25,966-file
library, every many-to-many that model expressed was 1:1 in the data.

- Ownership is a file. `GetFilePathsByRecordingMBIDs`,
  `LibraryMBIDIndex.CheckMBIDs`, `collectLibraryEntities` and
  `pruneStaleLocalCrossReferences` all join `audio_files`, so the 812
  orphaned recordings, 216 release groups and 260 artists that library
  carried are now structurally impossible.
- One projection: every track query selects from the `track_metadata`
  view, one row type, one mapper. Nine hand-rolled copies had drifted
  far enough to report different years on different screens.
- `library_id = 0` means every library, so each list query exists once
  instead of scoped and unscoped with a branch at every call site.
- No migration chain. `sql/schemas/` is the one description of the
  shape; `sql/migrations/`, `applyMigrations` and `schema_migrations`
  are squashed away, along with the drift between them that had sqlc
  generating against a stale schema.
- `database.InsertTestTrack` is the one test seeder; twenty test files
  had been assembling the old FK chain each in its own order.

## The catalog stores its ids as bytes

`explore_index`'s three 36-char MBID columns and its entity-type text
are 16 raw bytes and a small integer. The table and its six indexes go
780 MB to 405 MB on a real 2,052,200-row catalog, which is why a fresh
install is ~0.6 GB rather than ~1.0 GB.

- `backend/explore/mbid.go` is the only place the encoding is known;
  everything above it speaks dashed strings.
- `CHECK(length(mbid) = 16)` makes a stringly write fail at the insert
  rather than silently returning no rows, since SQLite does not coerce
  between TEXT and BLOB.
- The importer asks the artifact what encoding it carries and converts
  on the way in, so the artifact already published keeps working and no
  format bump is needed.
- `indexRowColumns`/`scanIndexRow` replace four copies of a 22-column
  list, and `TestStoredEncodingRoundTrips` sweeps every read path.

## An album page that says how much of the album is yours

- One question, asked once: is there a file. `filePaths` is filled by a
  single batched lookup when the tracklist settles, and the badge, the
  Play count, the dimmed rows and every menu item read it — replacing
  four claims of decreasing confidence that could show a green tick on
  an album whose every action did nothing.
- Play, Play 7 of 12, or no play button at all.
- `total_tracks` on `explore_index` (~2 bytes over 400,677 release
  groups) and on `audio_files` from tags that have always carried it:
  a complete MBID-matched album now makes no catalog call at all, where
  it used to spend the most expensive request the app makes.
- A merged cluster shows the running order the most releases agree on,
  and the version list marks the release you own rather than standing a
  synthetic entry in for it.
- `AlbumReleasesFailed`: a slow fetch is no longer reported as a failed
  one by a 12-second timer.
- Rows not in the library are dimmed in place (with `aria-disabled`)
  instead of the owned ones wearing a green tick and a legend.

## Caches and cover art get ceilings

- Only the three tiers of a cover are stored; the full-resolution copy
  nothing rendered was 1,134 MB of a 1.4 GB covers directory.
- One artist portrait is downloaded and the rest are remembered as
  URLs — 4.1 GB of a 5.3 GB cache was candidates no code path reads.
- `browsedArtBudget` and `httpCacheBudget` bound what an age cannot:
  the same install held art for 5,770 artists in a 1,301-artist
  library.
- `OrphanedArtistImagesJob` joined a bare MBID onto a sharded
  directory, so it deleted the rows that were the only record of the
  files it left behind. `explore.ArtistImageDir` is that layout's one
  definition now.

## The autotag queue asks whether there is work

`tagging_items` was a row per album folder, not a queue, and no query
read the `tag_status` column that held the answer. The four queue
queries ask the files, which matters most where it is least visible:
`startPrefetch` was scoring every album in a tagged library against
MusicBrainz.

## Phantom playlist tracks resolve in place

An M3U8 imported before its files leaves phantom rows; they now match
by path and fall back to position, keep their place in the playlist
when resolved, and pair best-first so two phantoms cannot claim the
same file.

## Playing a track plays the list it is in

Double-click, and Play on a single row's menu, queue the list as
displayed with `startIndex` on that row — the album page and the track
list used to queue one track and discard the album around it. A
multi-row selection still plays exactly itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AfVYUVExXsx1nSWrXN8mAh

.planning/NOTES.md

@@ -2354,3 +2354,43 @@ Five more things worth keeping:
   child does not prove the icon inside it is `pointer-events: none`.
   Both were checked with a real mouse (`mousemove`/`mousedown`/
   `mouseup`) and a real Tab/Enter before either was believed.
+
+## A queue of work has to ask whether there is work
+
+Reported as "the autotag page has every album in it, even the MB-tagged
+ones". Both halves of that were true and they were two different bugs,
+which is why the first answer found (the pending list) accounted for
+nine rows out of 2172.
+
+**Every scanned folder gets a `tagging_items` row.**
+`UpsertTaggingItemOnTrackAdd` runs per track with `status = 'pending'`
+and no condition, so the table is a row per album folder, not a queue.
+`saveAudioFile` *does* record the answer — `audio_files.tag_status` is
+`user_confirmed` on import for any file carrying a recording MBID, and
+`idx_audio_files_tag_status_untagged` was declared for the filter — but
+no query in the app read the column. Pending therefore meant "has a
+row". The four queue queries ask the files now
+(`EXISTS … tag_status = 'untagged'`), which matters most where it is
+least visible: `startPrefetch` was scoring every album in a tagged
+library against MusicBrainz.
+
+**The 2094 were the Completed section, and nobody completed them.** A
+backfill stamped `status = 'confirmed'` on every folder whose files
+already had MBIDs, and the sidebar keeps confirmed rows deliberately —
+so the review page's history became a list of the library. They are
+distinguishable without new state: every status flip the app performs
+goes through `SetTaggingItemStatus` or `SetTaggingItemBestMatch` and
+both stamp `last_checked_at`, so *confirmed with no `last_checked_at`,
+no score and no best match* is the backfill's row and not the user's.
+All 2094 were that shape; the two the app had actually applied were
+not. Migration 0007 sets `cleared_at` on them — the column exists for
+exactly this, and it keeps the row so a rescan cannot reset review
+state.
+
+Two smaller things fell out. The reviewed states are **exempt** from
+the untagged-files predicate, or an applied folder would vanish from
+Completed the instant it succeeded — the section is history, not work.
+And `tag_status` was only ever written by the *insert* path, so a file
+another tagger stamped after import kept `untagged` for ever and its
+folder kept asking; `updateAudioFile` promotes it now, guarded on
+`untagged` so a deliberate `user_skipped_permanent` survives a rescan.