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/mix.go

@@ -107,12 +107,11 @@ func (e *Service) mixSeedProfile(
 			artistCounts[artist.ArtistMbid]++
 			artistNames[artist.ArtistMbid] = artist.ArtistName
 		}
+	}
 
-		names, err := e.db.ReadQueries.GetGenreNamesByFilePath(ctx, p)
-		if err == nil {
-			for _, g := range names {
-				genres[g] = true
-			}
+	for _, names := range e.genresByPath(ctx, seedPaths) {
+		for _, g := range names {
+			genres[g] = true
 		}
 	}
 
@@ -148,6 +147,12 @@ func (e *Service) mixCandidates(
 
 	candidates := map[string]float64{}
 
+	// Every candidate path's genres, in one query.  This used to be a
+	// single-row lookup *per candidate* inside two nested loops -
+	// twenty seed artists by twenty similar artists by thirty paths is
+	// twelve thousand queries to assemble one mix.
+	pathGenres := e.genresByPath(ctx, e.similarArtistPaths(ctx, artistCounts))
+
 	for seedArtistMBID, count := range artistCounts {
 		similar, err := e.SimilarArtists(seedArtistMBID)
 		if err != nil {
@@ -178,13 +183,11 @@ func (e *Service) mixCandidates(
 					continue
 				}
 
-				if names, err := e.db.ReadQueries.GetGenreNamesByFilePath(ctx, p); err == nil {
-					for _, g := range names {
-						if seedGenres[g] {
-							weight += mixGenreBoost
+				for _, g := range pathGenres[p] {
+					if seedGenres[g] {
+						weight += mixGenreBoost
 
-							break
-						}
+						break
 					}
 				}
 
@@ -196,6 +199,64 @@ func (e *Service) mixCandidates(
 	return candidates
 }
 
+// genresByPath returns the genres of many files in one query.
+func (e *Service) genresByPath(
+	ctx context.Context, paths []string,
+) map[string][]string {
+	out := make(map[string][]string, len(paths))
+
+	if len(paths) == 0 {
+		return out
+	}
+
+	rows, err := e.db.ReadQueries.GetGenreNamesByFilePaths(ctx, paths)
+	if err != nil {
+		return out
+	}
+
+	for _, row := range rows {
+		out[row.FilePath] = append(out[row.FilePath], row.Name)
+	}
+
+	return out
+}
+
+// similarArtistPaths collects every owned file by an artist similar to
+// one of the seeds, so their genres can be fetched in one go.
+func (e *Service) similarArtistPaths(
+	ctx context.Context, artistCounts map[string]int,
+) []string {
+	var paths []string
+
+	for seedArtistMBID := range artistCounts {
+		similar, err := e.SimilarArtists(seedArtistMBID)
+		if err != nil {
+			continue
+		}
+
+		if len(similar) > mixSimilarArtistsPerSeed {
+			similar = similar[:mixSimilarArtistsPerSeed]
+		}
+
+		for _, sim := range similar {
+			if sim.ArtistMBID == "" {
+				continue
+			}
+
+			p, err := e.db.ReadQueries.GetFilePathsByArtistMBID(
+				ctx, sql.NullString{String: sim.ArtistMBID, Valid: true},
+			)
+			if err != nil {
+				continue
+			}
+
+			paths = append(paths, p...)
+		}
+	}
+
+	return paths
+}
+
 // weightedSample picks up to n distinct keys from weights without
 // replacement, biased toward higher weight (roulette-wheel selection).
 // A key with zero or negative weight is never picked.