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>

docs/hosting.md

@@ -119,8 +119,9 @@ to the music as well.
 
 ### Restoring
 
-A backup restores into a new Minstrel: an empty database, with no accounts
-and no library yet.
+A backup restores into a new Minstrel: one nobody has signed up to yet. If
+it has already scanned the music, that's fine; the backup's library takes
+the place of what the scan found.
 
 1. **Put the music back first**, at the path the library had. The restored
    settings name the old library folders, and each track is found again by
@@ -130,7 +131,9 @@ and no library yet.
 2. **Load the backup**, either way:
    - In the web app, choose **Create account**, then **Restore a backup**.
      Pick a `minstrel-….tar` file from the backup's `database/` folder and
-     enter the setup token from the log (see *First account*).
+     enter the setup token from the log (see *First account*). Minstrel
+     restarts by itself once the backup is in, so it runs on the restored
+     settings.
    - Or, with the server stopped, run
      `docker compose run --rm minstrel restore /backups/database/minstrel-20261102T030000Z.tar`,
      with the file on a volume the container can read. It brings the database