feat: a new Minstrel restores a backup, from the web app or the command line, then rescans (#6775)

A new server (no accounts, no library) offers "Restore a backup" beside
"Create account". The file goes up as the request body to
POST /api/auth/restore with the setup token from the log in X-Setup-Token,
since the backup's admins become this server's. It loads in one transaction,
then a library scan starts and finds each file at its path. The same load runs
offline as `minstrel restore FILE`, which migrates the schema first.

That path alone gets a 1 GiB, 15-minute body allowance instead of 4 MiB and
30s, and skips the JSON-only check for cookie writes: a stale cookie from the
old server would otherwise refuse the upload, and the custom header already
rules out a cross-site form.

The library sync answers 410 cursor_ahead to a cursor past the last one the
server has handed out, read from the change sequence, so an app synced with
the old server after the backup was made starts over instead of skipping the
ids the restored server reuses. Restore now leaves each sequence as having
issued the restored maximum, so that reading holds; a compacted log still
counts its handed-out cursors.

backup.Read keeps the underlying error in its chain, so an oversized upload
answers 413 rather than "not a backup". The hosting docs say how to put the
music back first and what is entered again afterwards.

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

docs/hosting.md

@@ -117,6 +117,34 @@ refusals in a row stop that night's music, since that is usually a key that
 may not write under `music/`. A bucket's upload limit applies
 to the music as well.
 
+### Restoring
+
+A backup restores into a new Minstrel: an empty database, with no accounts
+and no library yet.
+
+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
+   its path. If the backup includes the music, copy what is inside `music/`
+   into the library folder (with several library folders, into the folder
+   that holds them all).
+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*).
+   - 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
+     schema up to date first, so a backup from an older Minstrel loads into a
+     newer one; a backup from a newer Minstrel is refused.
+3. **Sign in** with the password you had. Everyone else's accounts, likes,
+   playlists and history are back too.
+
+The library scan then finds each file, and the artwork, tags, fingerprints
+and loudness are fetched or worked out again over the following hours. Keys
+for other services (ListenBrainz, AcoustID, artwork and tag providers,
+email, download clients) and the backup's own destination are entered again
+under **Admin**. An app that synced with the old server starts its sync over.
+
 ## Android app
 
 The app connects over whatever URL you give it, HTTP or HTTPS. Once the