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/sql/schemas/track_metadata.sql

@@ -1,23 +1,42 @@
-CREATE VIEW IF NOT EXISTS track_metadata AS
+-- The one definition of "a track, with everything a list needs".
+--
+-- A view is a definition, not data, so it is dropped and recreated on
+-- every open rather than carrying a migration alongside it: CREATE VIEW
+-- IF NOT EXISTS silently keeps an older database on the old definition,
+-- and a migration file restating it would be the second description of
+-- the schema the migration rules exist to prevent.
+--
+-- This projection used to exist **nine times** — four copies in
+-- audio_files.sql, two in playlists.sql, two in genres.sql, one in
+-- queue.sql — plus this view, which only the raw-SQL search paths used.
+-- They had already drifted: this view preferred the album's
+-- original_year for `year` and GetAllTracksWithFullMetadata used the
+-- track's own, so the same library reported different years on
+-- different screens.  Every query that wants a track row now selects
+-- from here, which is also why there is one row type and one mapper on
+-- the Go side instead of nine and a twenty-two-argument function.
+DROP VIEW IF EXISTS track_metadata;
+
+CREATE VIEW track_metadata AS
     SELECT
         af.id,
         af.file_path,
         af.length_milliseconds,
-        COALESCE(r.name, '') AS title,
-        COALESCE(ac.text, '') AS artist_name,
-        r.track_number,
-        r.disc_number,
-        COALESCE(rg.name, '') AS album,
+        af.title,
+        af.artist_credit AS artist_name,
+        af.track_number,
+        af.disc_number,
+        COALESCE(al.name, '') AS album,
         CAST(COALESCE(
             (SELECT GROUP_CONCAT(g.name, '||')
-             FROM recording_genres rg_sub
-             JOIN genres g ON rg_sub.genre_id = g.id
-             WHERE rg_sub.recording_id = r.id),
+             FROM file_genres fg
+             JOIN genres g ON g.id = fg.genre_id
+             WHERE fg.audio_file_id = af.id),
             ''
         ) AS TEXT) AS genre,
-        COALESCE(rg.original_year, rg.year, r.year, 0) AS year,
-        COALESCE(rg.year, r.year, 0) AS release_year,
-        COALESCE(r.composer, '') AS composer,
+        COALESCE(al.original_year, al.year, af.year, 0) AS year,
+        COALESCE(al.year, af.year, 0) AS release_year,
+        af.composer,
         COALESCE(ft.extension, '') AS file_type,
         af.sample_rate,
         af.bit_depth,
@@ -28,20 +47,13 @@ CREATE VIEW IF NOT EXISTS track_metadata AS
         af.play_count,
         af.last_played,
         COALESCE(ca.file_path, '') AS cover_art_path,
-        COALESCE(a.mbid, '') AS artist_mbid,
-        COALESCE(rg.mbid, '') AS release_group_mbid,
-        COALESCE(r.mbid, '') AS recording_mbid
+        COALESCE(ar.mbid, '') AS artist_mbid,
+        COALESCE(al.mbid, '') AS release_group_mbid,
+        COALESCE(af.recording_mbid, '') AS recording_mbid,
+        af.album_id,
+        af.artist_id
     FROM audio_files af
-    LEFT JOIN recordings r ON af.recording_id = r.id
-    LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id
-    LEFT JOIN artist_credit_artist aca ON aca.credit_id = ac.id
-    LEFT JOIN artists a ON a.id = aca.artist_id
-    LEFT JOIN (
-        SELECT recording_id,
-            MIN(release_group_id) AS release_group_id
-        FROM release_group_recordings
-        GROUP BY recording_id
-    ) rgr ON r.id = rgr.recording_id
-    LEFT JOIN release_groups rg ON rgr.release_group_id = rg.id
-    LEFT JOIN cover_art ca ON rg.cover_art_id = ca.id
-    LEFT JOIN file_types ft ON af.file_type_id = ft.id;
+    LEFT JOIN albums al     ON al.id = af.album_id
+    LEFT JOIN artists ar    ON ar.id = af.artist_id
+    LEFT JOIN cover_art ca  ON ca.id = al.cover_art_id
+    LEFT JOIN file_types ft ON ft.id = af.file_type_id;