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

backend/database/explorefts_test.go

@@ -1,18 +1,26 @@
 package database
 
 import (
+	"crypto/sha256"
 	"strings"
 	"testing"
 )
 
 // seedExploreRow inserts one explore_index row.
+//
+// The catalog stores an MBID as 16 raw bytes and an entity type as a
+// code (see backend/explore/mbid.go), and the column says so, so the
+// label these tests use as an id is hashed into something the table
+// will accept.  What they actually assert on is the FTS text.
 func seedExploreRow(t *testing.T, db *DB, mbid, title, artist string) {
 	t.Helper()
 
+	sum := sha256.Sum256([]byte(mbid))
+
 	if _, err := db.ExecContext(`
 		INSERT INTO explore_index (entity_type, mbid, title, artist_name, artist_mbid)
-		VALUES ('recording', ?, ?, ?, '')
-	`, mbid, title, artist); err != nil {
+		VALUES (3 /* recording */, ?, ?, ?, x'')
+	`, sum[:16], title, artist); err != nil {
 		t.Fatalf("seed %s: %v", mbid, err)
 	}
 }
@@ -202,7 +210,8 @@ func TestExploreFTSUpdateSkipsUnchangedText(t *testing.T) {
 
 	// A popularity refresh: an FTS column is not named at all.
 	if _, err := db.ExecContext(
-		"UPDATE explore_index SET popularity = 42 WHERE mbid = 'mbid-1'",
+		"UPDATE explore_index SET popularity = 42 WHERE title = ?",
+		"Unchanged Title",
 	); err != nil {
 		t.Fatalf("popularity update: %v", err)
 	}
@@ -212,8 +221,8 @@ func TestExploreFTSUpdateSkipsUnchangedText(t *testing.T) {
 	if _, err := db.ExecContext(`
 		UPDATE explore_index
 		SET title = 'Unchanged Title', artist_name = 'Steady Artist', popularity = 43
-		WHERE mbid = 'mbid-1'
-	`); err != nil {
+		WHERE title = ?
+	`, "Unchanged Title"); err != nil {
 		t.Fatalf("no-op text update: %v", err)
 	}
 
@@ -237,7 +246,8 @@ func TestExploreFTSUpdateReindexesChangedText(t *testing.T) {
 	seedExploreRow(t, db, "mbid-2", "Original Title", "Some Artist")
 
 	if _, err := db.ExecContext(
-		"UPDATE explore_index SET title = 'Corrected Title' WHERE mbid = 'mbid-2'",
+		"UPDATE explore_index SET title = 'Corrected Title' WHERE title = ?",
+		"Original Title",
 	); err != nil {
 		t.Fatalf("rename: %v", err)
 	}
@@ -252,7 +262,8 @@ func TestExploreFTSUpdateReindexesChangedText(t *testing.T) {
 
 	// The same for the other two indexed columns.
 	if _, err := db.ExecContext(
-		"UPDATE explore_index SET artist_name = 'Renamed Artist', aliases = 'AKA Thing' WHERE mbid = 'mbid-2'",
+		"UPDATE explore_index SET artist_name = 'Renamed Artist', aliases = 'AKA Thing' WHERE title = ?",
+		"Corrected Title",
 	); err != nil {
 		t.Fatalf("artist rename: %v", err)
 	}