fix: a new server takes a backup after its first scan, and restarts to load the restored settings (#6786)

A one-off end-to-end run of the :dev image (scratch Postgres, generated
FLAC files, no outbound network) found that the web restore, done the way
docs/hosting.md describes, was refused with not_empty: a new server's
startup scan fills the library before anyone signs up, and Restore counted
tracks as occupied. The command-line restore passed the same run.

- Restore now asks only that the instance has no accounts. The library the
  first scan found is replaced by the backup's (every foreign key into
  artists/albums/tracks cascades or sets null), and the restore locks the
  library tables so a running scan waits rather than adding to them.
- Settings are read once at boot by about ten services, the library folders
  among them, so after a web restore the server ran on its defaults until
  restarted. The restore endpoint now answers, then asks the process to
  restart in place (syscall.Exec of the same binary, arguments and
  environment), which needs no container restart policy. The startup scan
  of the restarted server replaces the scan the endpoint used to start.
- The restore form says Minstrel is restarting; the docs say a scanned new
  server still takes a backup.

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

web/src/lib/auth/store.svelte.ts

@@ -75,7 +75,7 @@ export async function register(opts: {
   void sendTimezoneIfStale();
 }
 
-export type RestoreResult = { accounts: number; scan_started: boolean };
+export type RestoreResult = { accounts: number; restarting: boolean };
 
 /**
  * Loads a backup file into a new server, which has no accounts yet. The file