feat: the backup can include the music, sending each library file once to music/ and again only when it changes (#6774)

With "Include the music" on, each night's run sends the library files the
destination doesn't hold yet, after the database file has landed, for up to
six hours; a large library spans several nights the first time. Files keep
their path under the library folders' common root, beside database/ in the
same folder or bucket.

backup_music_files records each file's size and mtime as sent, so a night
sends only new and changed files, and a rescan that touches a track but finds
the same file sends nothing. A changed file goes to its old name; Minstrel
never deletes there. An object the bucket refuses (a locked overwrite) is
kept with its answer and goes again once the file changes or the settings are
saved; three refusals in a row stop the night's music, since that is a key
that may not write under music/. A folder copy is written under a .part name
and renamed, keeping the file's time.

The panel shows how much of the library is in, the refused files, and each
run's music count and note. The verdict still turns on the database file.

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

internal/db/queries/backups.sql

@@ -14,6 +14,7 @@ UPDATE backup_settings
        secret_key  = $7,
        run_hour    = $8,
        upload_limit_kib = $9,
+       include_music = $10,
        updated_at  = now()
  WHERE id = true;
 
@@ -27,11 +28,15 @@ UPDATE backup_runs
        finished_at = now(),
        location    = $3,
        bytes       = $4,
-       error       = $5
+       error       = $5,
+       music_files = $6,
+       music_bytes = $7,
+       music_note  = $8
  WHERE id = $1;
 
 -- name: ListBackupRuns :many
-SELECT id, trigger, status, started_at, finished_at, location, bytes, error
+SELECT id, trigger, status, started_at, finished_at, location, bytes, error,
+       music_files, music_bytes, music_note
   FROM backup_runs
  ORDER BY started_at DESC
  LIMIT $1;
@@ -59,3 +64,47 @@ UPDATE backup_runs
 
 -- name: PruneBackupRuns :execrows
 DELETE FROM backup_runs WHERE status <> 'running' AND started_at < $1;
+
+-- name: PendingBackupMusic :many
+-- Library files the backup may not hold as they are now, in path order from
+-- after_path, so a night that runs out of time carries on where it stopped:
+-- never sent, a different size, touched by a scan since they were last
+-- checked, or refused before the settings were last saved (a fixed key gets
+-- another go). The caller compares the file's own size and mtime before
+-- sending.
+SELECT t.id, t.file_path, t.file_size,
+       m.size AS sent_size, m.mtime AS sent_mtime, m.error AS sent_error
+  FROM tracks t
+  LEFT JOIN backup_music_files m ON m.track_id = t.id
+ WHERE t.missing_since IS NULL
+   AND t.file_path > sqlc.arg(after_path)::text
+   AND (m.track_id IS NULL OR m.size <> t.file_size OR t.updated_at > m.checked_at
+        OR (m.error <> '' AND m.checked_at < (SELECT updated_at FROM backup_settings)))
+ ORDER BY t.file_path
+ LIMIT sqlc.arg(max_rows);
+
+-- name: RecordBackupMusic :exec
+INSERT INTO backup_music_files (track_id, size, mtime, location, error)
+VALUES ($1, $2, $3, $4, $5)
+ON CONFLICT (track_id) DO UPDATE
+   SET size = EXCLUDED.size, mtime = EXCLUDED.mtime, location = EXCLUDED.location,
+       error = EXCLUDED.error, checked_at = now();
+
+-- name: BackupMusicProgress :one
+-- How much of the library the backup holds as it is now.
+SELECT count(*)::bigint AS total_files,
+       COALESCE(sum(t.file_size), 0)::bigint AS total_bytes,
+       count(*) FILTER (WHERE m.error = '' AND m.size = t.file_size AND t.updated_at <= m.checked_at)::bigint AS sent_files,
+       COALESCE(sum(t.file_size) FILTER (WHERE m.error = '' AND m.size = t.file_size AND t.updated_at <= m.checked_at), 0)::bigint AS sent_bytes,
+       count(*) FILTER (WHERE m.error <> '' AND m.size = t.file_size AND t.updated_at <= m.checked_at)::bigint AS refused_files
+  FROM tracks t
+  LEFT JOIN backup_music_files m ON m.track_id = t.id
+ WHERE t.missing_since IS NULL;
+
+-- name: RefusedBackupMusic :many
+SELECT t.file_path, m.error, m.checked_at
+  FROM backup_music_files m
+  JOIN tracks t ON t.id = m.track_id
+ WHERE m.error <> '' AND m.size = t.file_size AND t.updated_at <= m.checked_at
+ ORDER BY m.checked_at DESC
+ LIMIT $1;