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/playlist/match_test.go

@@ -409,3 +409,122 @@ func stringSliceEqual(a, b []string) bool {
 
 	return true
 }
+
+// TestScoreCandidateUnknownCandidateDuration pins which side may be the
+// one that does not know a duration.  An M3U8 without EXTINF lines was
+// already handled; a *library* file whose length was never read was not,
+// so an otherwise exact match was scored out of 0.9 and could not reach
+// the auto-match threshold.
+func TestScoreCandidateUnknownCandidateDuration(t *testing.T) {
+	t.Parallel()
+
+	pp := newPhantomProfile(
+		"/old/Music/Artist/01 - Song.mp3",
+		"Artist - Song",
+		240,
+	)
+
+	score := scoreCandidate(
+		pp,
+		"/new/Music/Artist/01 - Song.mp3",
+		"Song",
+		"Artist",
+		0, // the library never read this file's length
+	)
+
+	if score < autoMatchMinimum {
+		t.Errorf(
+			"score = %f, want >= %f for an exact match with no "+
+				"candidate duration",
+			score, autoMatchMinimum,
+		)
+	}
+}
+
+// TestAssignBestFirst covers the rule that one library file cannot
+// resolve two phantom tracks, and which phantom gets it when both want
+// the same one.
+func TestAssignBestFirst(t *testing.T) {
+	t.Parallel()
+
+	offers := []phantomOffer{
+		// The playlist's first phantom wants this file, but only
+		// just — and the third one is a better answer for it.
+		{
+			phantomPath: "/gone/a.mp3",
+			candidate: CandidateTrack{
+				FilePath: "/lib/shared.mp3", Score: 0.86,
+			},
+		},
+		{
+			phantomPath: "/gone/b.mp3",
+			candidate: CandidateTrack{
+				FilePath: "/lib/b.mp3", Score: 0.90,
+			},
+		},
+		{
+			phantomPath: "/gone/c.mp3",
+			candidate: CandidateTrack{
+				FilePath: "/lib/shared.mp3", Score: 0.98,
+			},
+		},
+	}
+
+	matches := assignBestFirst(offers)
+
+	if len(matches) != 2 {
+		t.Fatalf("matches = %d, want 2", len(matches))
+	}
+
+	got := make(map[string]string, len(matches))
+	for _, m := range matches {
+		got[m.PhantomPath] = m.Candidate.FilePath
+	}
+
+	if got["/gone/c.mp3"] != "/lib/shared.mp3" {
+		t.Errorf(
+			"the shared file went to %v, want /gone/c.mp3 to have it",
+			got,
+		)
+	}
+
+	if _, claimed := got["/gone/a.mp3"]; claimed {
+		t.Error("/gone/a.mp3 took a file a better match had claimed")
+	}
+
+	if got["/gone/b.mp3"] != "/lib/b.mp3" {
+		t.Errorf("/gone/b.mp3 matched %q, want /lib/b.mp3", got["/gone/b.mp3"])
+	}
+}
+
+// TestAssignBestFirstKeepsOnePerPhantom: a phantom with several
+// confident candidates takes its best one and no more.
+func TestAssignBestFirstKeepsOnePerPhantom(t *testing.T) {
+	t.Parallel()
+
+	matches := assignBestFirst([]phantomOffer{
+		{
+			phantomPath: "/gone/a.mp3",
+			candidate: CandidateTrack{
+				FilePath: "/lib/one.mp3", Score: 0.90,
+			},
+		},
+		{
+			phantomPath: "/gone/a.mp3",
+			candidate: CandidateTrack{
+				FilePath: "/lib/two.mp3", Score: 0.95,
+			},
+		},
+	})
+
+	if len(matches) != 1 {
+		t.Fatalf("matches = %d, want 1", len(matches))
+	}
+
+	if matches[0].Candidate.FilePath != "/lib/two.mp3" {
+		t.Errorf(
+			"matched %q, want the higher-scoring /lib/two.mp3",
+			matches[0].Candidate.FilePath,
+		)
+	}
+}