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

frontend/src/components/track-list/track-list.ts

@@ -1242,12 +1242,32 @@ export class TrackList
      *  has existed in the defaults and in Settings since it was written
      *  and has never had anything on the other end of it. */
     private handleShortcutPlay = (): void => {
-        const filePaths = this.selection.getSelectedKeysOrdered();
+        this.playSelection(this.selection.getSelectedKeysOrdered());
+    };
 
+    /**
+     * "Play" means the same thing from the menu and from Enter, and it
+     * asks how much the user selected. One row is a position in the
+     * list - it queues the list from there, exactly as double-clicking
+     * does. Several rows are an explicit choice of *those* tracks, so
+     * they become the queue on their own (and `shuffleStart` applies,
+     * since no one row was named as the place to start).
+     */
+    private playSelection(filePaths: string[]): void {
         if (filePaths.length === 0) return;
 
+        if (filePaths.length === 1) {
+            const index = this.displayIndexOf(filePaths[0]!);
+
+            if (index >= 0) {
+                this.playFromRow(index);
+
+                return;
+            }
+        }
+
         queueStore.setQueue(filePaths, 0, true, this.effectiveQueueSource);
-    };
+    }
 
     override willUpdate(
         changed: Map<PropertyKey, unknown>,
@@ -1544,7 +1564,7 @@ export class TrackList
     private onDelegatedDblClick = (e: MouseEvent) => {
         const hit = this.resolveTrackFromEvent(e);
 
-        if (hit) this.onTrackRowDblClick(hit.track);
+        if (hit) this.onTrackRowDblClick(hit.track, hit.index);
     };
 
     private onDelegatedContextMenu = (e: MouseEvent) => {
@@ -1574,9 +1594,45 @@ export class TrackList
         this.selection.handleItemClick(e, track.FilePath, index);
     }
 
-    private onTrackRowDblClick(track: library.Track) {
+    private onTrackRowDblClick(_track: library.Track, index: number) {
         this.selection.clear();
-        queueStore.setQueue([track.FilePath], 0, false, this.effectiveQueueSource);
+        this.playFromRow(index);
+    }
+
+    /**
+     * Activating one row plays the list that row is in, from that row -
+     * the library, the artist or the genre the user is looking at, not
+     * a queue of one. The paths come from `cachedSortedTracks`, so it
+     * is the list as *displayed*: whatever the current sort, search and
+     * library filter have made of it, which is the only order the user
+     * can see and therefore the only one they can mean.
+     */
+    private playFromRow(index: number) {
+        const filePaths = this.cachedSortedTracks.map(
+            (t) => t.FilePath,
+        );
+
+        if (filePaths.length === 0 || index < 0) return;
+
+        queueStore.setQueue(
+            filePaths,
+            index,
+            false,
+            this.effectiveQueueSource,
+        );
+    }
+
+    /**
+     * Where a "play this" command lands in the displayed list, or -1.
+     *
+     * Selection keys are file paths, which survive the re-sorts and
+     * refetches an index does not - so the index is looked up at the
+     * moment it is used rather than remembered.
+     */
+    private displayIndexOf(filePath: string): number {
+        return this.cachedSortedTracks.findIndex(
+            (t) => t.FilePath === filePath,
+        );
     }
 
     private onTrackContextMenu(e: MouseEvent, track: library.Track) {
@@ -1648,7 +1704,7 @@ export class TrackList
 
         switch (action) {
             case 'play':
-                queueStore.setQueue(filePaths, 0, true, this.effectiveQueueSource);
+                this.playSelection(filePaths);
                 break;
             case 'add-to-queue':
                 queueStore.addTracksToQueue(filePaths);