fix: a fresh app install syncs the whole library, not the last 30 days of changes (#6778)

The change feed dropped every row older than 30 days, and an app syncing
from cursor 0 builds its library from the feed, so a fresh install, cleared
data or a 410 reset got only what had changed lately.

The compactor now keeps each entity's latest change however old. It drops a
row once a newer one for the same entity exists, and a delete once it is 30
days old, raising a stored mark (library_changes_trimmed) past it; an app
whose cursor is below the mark is sent back to 0, as before. Migration 0101
adds the index the compactor needs, seeds the mark where the old compactor
left off, and gives every artist, album and track whose latest row is not an
upsert a fresh one, so the rows the old compactor dropped come back. The
backup carries the mark with the feed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/dbq/library_changes.sql.go

@@ -11,15 +11,38 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-const deleteOldLibraryChanges = `-- name: DeleteOldLibraryChanges :execrows
-DELETE FROM library_changes WHERE changed_at < $1
+const deleteOldLibraryDeletes = `-- name: DeleteOldLibraryDeletes :one
+WITH gone AS (
+    DELETE FROM library_changes WHERE op = 'delete' AND changed_at < $1 RETURNING id
+), mark AS (
+    UPDATE library_changes_trimmed
+       SET through = GREATEST(through, (SELECT COALESCE(max(id), 0) FROM gone))
+)
+SELECT count(*)::BIGINT FROM gone
 `
 
-// Removes library_changes rows older than the given cutoff. Run from
-// the sync compactor goroutine on a daily tick. Returns the number of
-// rows deleted so the worker can log meaningful progress.
-func (q *Queries) DeleteOldLibraryChanges(ctx context.Context, changedAt pgtype.Timestamptz) (int64, error) {
-	result, err := q.db.Exec(ctx, deleteOldLibraryChanges, changedAt)
+// Removes deletes older than the cutoff and raises the trimmed mark past the
+// highest one, so an app whose cursor is below it starts over. Returns how
+// many went.
+func (q *Queries) DeleteOldLibraryDeletes(ctx context.Context, changedAt pgtype.Timestamptz) (int64, error) {
+	row := q.db.QueryRow(ctx, deleteOldLibraryDeletes, changedAt)
+	var column_1 int64
+	err := row.Scan(&column_1)
+	return column_1, err
+}
+
+const deleteSupersededLibraryChanges = `-- name: DeleteSupersededLibraryChanges :execrows
+DELETE FROM library_changes old
+ WHERE EXISTS (SELECT 1 FROM library_changes newer
+                WHERE newer.entity_type = old.entity_type
+                  AND newer.entity_id = old.entity_id
+                  AND newer.id > old.id)
+`
+
+// Removes every row a newer row for the same entity has superseded. An app
+// reading from any cursor still meets the newer row, so it loses nothing.
+func (q *Queries) DeleteSupersededLibraryChanges(ctx context.Context) (int64, error) {
+	result, err := q.db.Exec(ctx, deleteSupersededLibraryChanges)
 	if err != nil {
 		return 0, err
 	}
@@ -79,6 +102,19 @@ func (q *Queries) GetLibraryChangesSince(ctx context.Context, arg GetLibraryChan
 	return items, nil
 }
 
+const getLibraryChangesTrimmedThrough = `-- name: GetLibraryChangesTrimmedThrough :one
+SELECT through FROM library_changes_trimmed
+`
+
+// The highest id of a delete the compactor has dropped. A cursor below it may
+// have missed that delete.
+func (q *Queries) GetLibraryChangesTrimmedThrough(ctx context.Context) (int64, error) {
+	row := q.db.QueryRow(ctx, getLibraryChangesTrimmedThrough)
+	var through int64
+	err := row.Scan(&through)
+	return through, err
+}
+
 const getMaxLibraryChangeCursor = `-- name: GetMaxLibraryChangeCursor :one
 SELECT COALESCE(MAX(id), 0)::BIGINT FROM library_changes
 `
@@ -90,17 +126,6 @@ func (q *Queries) GetMaxLibraryChangeCursor(ctx context.Context) (int64, error)
 	return column_1, err
 }
 
-const getMinLibraryChangeCursor = `-- name: GetMinLibraryChangeCursor :one
-SELECT COALESCE(MIN(id), 0)::BIGINT FROM library_changes
-`
-
-func (q *Queries) GetMinLibraryChangeCursor(ctx context.Context) (int64, error) {
-	row := q.db.QueryRow(ctx, getMinLibraryChangeCursor)
-	var column_1 int64
-	err := row.Scan(&column_1)
-	return column_1, err
-}
-
 const insertLibraryChange = `-- name: InsertLibraryChange :exec
 INSERT INTO library_changes (entity_type, entity_id, op)
 VALUES ($1, $2, $3)