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>

internal/api/api.go

@@ -99,6 +99,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		api.Post("/auth/login", h.handleLogin)
 		api.Post("/auth/register", h.handleRegister)
 		api.Get("/auth/setup-status", h.handleSetupStatus)
+		api.Post("/auth/restore", h.handleRestore)
 		api.Post("/auth/forgot-password", h.handleForgotPassword)
 		api.Post("/auth/reset-password", h.handleResetPassword)