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

web/src/lib/components/auth/RestoreForm.svelte

@@ -22,7 +22,7 @@
       case 'newer_backup':
         return 'That backup was made by a newer Minstrel. Update this server first, then restore it.';
       case 'not_empty':
-        return 'This server already has accounts or a library, so it can’t take a backup.';
+        return 'This server already has accounts, so it can’t take a backup.';
       case 'too_large':
         return 'That file is larger than a backup can be.';
       default:
@@ -50,12 +50,13 @@
 {#if done}
   <div class="space-y-3" role="status" data-testid="restore-done">
     <p class="text-sm text-text-primary">
-      Restored {plural(done.accounts, 'account', 'accounts')}. Sign in with the password you had.
+      Restored {plural(done.accounts, 'account', 'accounts')}.
+      {done.restarting
+        ? 'Minstrel is restarting to load them; in a few seconds, sign in with the password you had.'
+        : 'Sign in with the password you had.'}
     </p>
     <p class="text-sm text-text-secondary">
-      {done.scan_started
-        ? 'The library scan has started: it finds each music file and fetches the artwork and tags again.'
-        : 'Start a library scan under Admin › Library once you’re signed in.'}
+      The library scan finds each music file and fetches the artwork and tags again.
     </p>
     <a href="/login" class="block w-full rounded bg-accent px-3 py-2 text-center font-medium text-background">Sign in</a>
   </div>

web/src/routes/register/register.test.ts

@@ -166,7 +166,7 @@ describe('/register page', () => {
 
   test('a new server restores a backup with the setup token, then sends you to sign in', async () => {
     (getSetupStatus as ReturnType<typeof vi.fn>).mockResolvedValue({ setup_required: true });
-    (restoreBackup as ReturnType<typeof vi.fn>).mockResolvedValue({ accounts: 2, scan_started: true });
+    (restoreBackup as ReturnType<typeof vi.fn>).mockResolvedValue({ accounts: 2, restarting: true });
     render(RegisterPage);
     await fireEvent.click(await screen.findByRole('button', { name: /restore a backup/i }));
     expect(screen.getByRole('heading', { name: /restore a backup/i })).toBeInTheDocument();
@@ -179,8 +179,8 @@ describe('/register page', () => {
 
     await waitFor(() => expect(restoreBackup).toHaveBeenCalledWith(file, '0123abcd'));
     const done = await screen.findByTestId('restore-done');
-    expect(done.textContent).toMatch(/Restored 2 accounts\. Sign in with the password you had\./);
-    expect(done.textContent).toMatch(/library scan has started/);
+    expect(done.textContent).toMatch(/Restored 2 accounts\.\s+Minstrel is restarting to load them; in a few seconds, sign in with the password you had\./);
+    expect(done.textContent).toMatch(/library scan finds each music file/);
     expect(within(done).getByRole('link', { name: /^sign in$/i })).toHaveAttribute('href', '/login');
   });