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/explore/dumpimport_test.go

@@ -277,6 +277,12 @@ func canonicalDataCSV(t *testing.T) []byte {
 			"3", "11", "{" + artA + "," + artB + "}", "Solo Star feat. Guest",
 			relB, "Duet Album", recC, "Duet Song", "x", "1",
 		},
+		// Unplayed, so it survives no popularity floor and is not
+		// indexed -- but it is still a track on Big Album.
+		{
+			"4", "10", "{" + artA + "}", "Solo Star",
+			relA, "Big Album", recD, "Album Filler", "x", "1",
+		},
 	}
 
 	return csvBytes(t, rows)
@@ -594,7 +600,7 @@ func TestDumpImportEndToEnd(t *testing.T) {
 
 		rows, err := db.QueryContext(
 			"SELECT title, popularity FROM explore_index WHERE mbid = ? AND entity_type = ?",
-			mbid, entityType,
+			dbMBID(mbid), dbEntityType(entityType),
 		)
 		if err != nil {
 			t.Fatalf("query: %v", err)
@@ -627,11 +633,51 @@ func TestDumpImportEndToEnd(t *testing.T) {
 	assertRow(rgB, "release_group", "Duet Album", 12)
 	assertRow(artA, "artist", "Solo Star", 35)
 
+	// The per-release-group denominator: how many tracks the release
+	// has, counted from the canonical dump *before* the popularity
+	// filter.  Big Album has three, one of which nobody has played and
+	// which is therefore not indexed as a recording at all -- a
+	// denominator built from the kept recordings would say two, and
+	// "you have 2 of 2" about a three-track album is worse than saying
+	// nothing.
+	assertTotalTracks := func(mbid string, want int) {
+		t.Helper()
+
+		var got int
+
+		if err := db.QueryRowWriter(
+			"SELECT total_tracks FROM explore_index WHERE mbid = ? AND entity_type = 2",
+			dbMBID(mbid),
+		).Scan(&got); err != nil {
+			t.Fatalf("read total_tracks for %s: %v", mbid, err)
+		}
+
+		if got != want {
+			t.Errorf("total_tracks for %s = %d, want %d", mbid, got, want)
+		}
+	}
+
+	assertTotalTracks(rgA, 3)
+	assertTotalTracks(rgB, 1)
+
+	// And the unplayed track is still not an indexed recording.
+	var fillerRows int
+
+	if err := db.QueryRowWriter(
+		"SELECT COUNT(*) FROM explore_index WHERE mbid = ?", dbMBID(recD),
+	).Scan(&fillerRows); err != nil {
+		t.Fatalf("count recD rows: %v", err)
+	}
+
+	if fillerRows != 0 {
+		t.Errorf("unplayed track was indexed as a recording (%d rows)", fillerRows)
+	}
+
 	// artB only ever appears in a multi-artist credit: no name is
 	// derivable from the dump, so it must be queued for the API
 	// metadata patch instead of being written nameless.
 	rows, err := db.QueryContext(
-		"SELECT COUNT(*) FROM explore_index WHERE mbid = ?", artB,
+		"SELECT COUNT(*) FROM explore_index WHERE mbid = ?", dbMBID(artB),
 	)
 	if err != nil {
 		t.Fatalf("query artB: %v", err)
@@ -1006,7 +1052,7 @@ func TestListenerCountUpdateDoesNotTouchPopularity(t *testing.T) {
 	}
 
 	rows, err := db.QueryContext(
-		"SELECT popularity, listener_count FROM explore_index WHERE mbid = ?", recA,
+		"SELECT popularity, listener_count FROM explore_index WHERE mbid = ?", dbMBID(recA),
 	)
 	if err != nil {
 		t.Fatalf("query: %v", err)