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/artifactimport.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	_ "modernc.org/sqlite" // SQLite driver for reading the artifact file.
@@ -91,9 +92,90 @@ type artifactInfo struct {
 // TestArtifactColumnsMatchExporter.
 const artifactCatalogColumns = `entity_type, mbid, title, artist_name, artist_mbid,
 	aliases, popularity, listener_count, duration, caa_release_mbid,
-	release_name, primary_type, secondary_types, release_date,
+	release_name, primary_type, secondary_types, release_date, total_tracks,
 	artist_type, country, disambiguation, sort_name, discog_fetched`
 
+// artifactSelectColumns is the same list as read *from an artifact*,
+// converting the two columns whose storage this app changed.
+//
+// The local table stores an MBID as 16 raw bytes and an entity type as a
+// small integer, which took the catalog and its indexes from 677 MB to
+// 389 MB.  A published artifact still carries the text form, and there
+// is no reason it should not: converting on the way in costs one
+// `unhex` per row on a once-a-month import, and it means a new build
+// reads the artifact that is already out there rather than requiring
+// one to be rebuilt and re-downloaded first.
+//
+// An artifact that already carries the compact form is copied straight
+// through - `artifactStoresText` decides which, by asking the artifact
+// rather than by trusting a version number.
+func artifactSelectColumns(text, totals bool) string {
+	totalTracks := "total_tracks"
+	if !totals {
+		// An artifact built before the column existed. Zero is what the
+		// column means by "the catalog does not say", so an older
+		// artifact imports as one that declines to answer rather than
+		// failing to import at all.
+		totalTracks = "0"
+	}
+
+	if !text {
+		return strings.Replace(
+			artifactCatalogColumns, "total_tracks", totalTracks, 1,
+		)
+	}
+
+	return `CASE entity_type
+			WHEN 'artist' THEN 1
+			WHEN 'release_group' THEN 2
+			WHEN 'recording' THEN 3
+			ELSE 0 END,
+		unhex(replace(mbid, '-', '')),
+		title, artist_name,
+		CASE WHEN artist_mbid = '' THEN x''
+			ELSE unhex(replace(artist_mbid, '-', '')) END,
+		aliases, popularity, listener_count, duration,
+		CASE WHEN caa_release_mbid = '' THEN x''
+			ELSE unhex(replace(caa_release_mbid, '-', '')) END,
+		release_name, primary_type, secondary_types, release_date, ` +
+		totalTracks + `,
+		artist_type, country, disambiguation, sort_name, discog_fetched`
+}
+
+// artifactHasTotals reports whether the attached artifact carries the
+// per-release-group track denominator.  An artifact published before
+// that column existed is still a perfectly good catalog, so it is asked
+// rather than assumed - the same rule, and the same handle, as
+// artifactStoresText below.
+func (si *SearchIndex) artifactHasTotals() bool {
+	var n int
+
+	err := si.db.QueryRowWriter(
+		`SELECT COUNT(*) FROM pragma_table_info('explore_index', 'core')
+		 WHERE name = 'total_tracks'`,
+	).Scan(&n)
+
+	return err == nil && n > 0
+}
+
+// artifactStoresText reports whether the attached artifact carries the
+// old text encoding.
+func (si *SearchIndex) artifactStoresText() bool {
+	// The writer, not QueryContext: "core" is attached to that one
+	// connection and does not exist on the read pool.  Asking the wrong
+	// handle errors, and the fallback would then convert an artifact
+	// that needs no conversion.
+	var kind string
+
+	if err := si.db.QueryRowWriter(
+		"SELECT typeof(mbid) FROM core.explore_index LIMIT 1",
+	).Scan(&kind); err != nil {
+		return true
+	}
+
+	return kind == "text"
+}
+
 // inspectArtifact opens the artifact read-only and reports what it
 // declares, without touching the live index.  Validation happens here so
 // a bad download is rejected before anything is attached.
@@ -263,9 +345,13 @@ func (si *SearchIndex) analyzeIndex() {
 // is an index range scan and a cancelled import leaves committed work
 // behind rather than rolling it all back.
 func (si *SearchIndex) mergeArtifactRows(ctx context.Context, total int) (int, error) {
+	selectColumns := artifactSelectColumns(
+		si.artifactStoresText(), si.artifactHasTotals(),
+	)
+
 	insertSQL := `
 		INSERT INTO explore_index (` + artifactCatalogColumns + `)
-		SELECT ` + artifactCatalogColumns + `
+		SELECT ` + selectColumns + `
 		FROM core.explore_index
 		WHERE mbid > ?` + upsertIndexConflictSQL
 
@@ -273,7 +359,7 @@ func (si *SearchIndex) mergeArtifactRows(ctx context.Context, total int) (int, e
 	// appended only while one exists.
 	insertRangeSQL := `
 		INSERT INTO explore_index (` + artifactCatalogColumns + `)
-		SELECT ` + artifactCatalogColumns + `
+		SELECT ` + selectColumns + `
 		FROM core.explore_index
 		WHERE mbid > ? AND mbid <= ?` + upsertIndexConflictSQL