First-time setup: the first login made while no administrator exists becomes it

Sign-up stays closed (invitations only), so a new server's first login could only be made over SSH with admin
create-root, and an invitation needs a group, which needs a login. While no administrator exists, GET
/clientapi/user/setup says so and POST takes a SetupForm (login, password, first persona): the login is made the
administrator with its persona, and setup closes for good (owner decision 2026-10-16: first come, first served, no setup
code).

ServerSetup is one document with a fixed id, so the first request to insert it wins and two setups at once make one
administrator. A setup that fails before its login is made gives the claim back; one that named an administrator never
does, whoever is demoted or deleted later, and the CLI stays the way back. admin promote and create-root --admin seal it
too, migration _018 seals a server that already had an administrator, and no backup holds it nor does a restore drop it,
since a restore tombstones an administrator made after the backup. Public sign-up, NodeInfo and the instance API are
unchanged. RootUser.SetAdmin is the one place an administrator is made (with isModerator, which IsAdminPolicy asks for),
and the invitation endpoints share InsertPersona with setup.

Tests: ServerSetupTests on the invitations-only host (setup, the race, failed setups, sign-up still closed, the CLI's
seal) and the migration; the whole suite passes (962). In the pasture, decePub's setup and invitation pages pass end to
end.

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

PrivaPub/Infrastructure/Backup/ServerBackup.cs

@@ -40,7 +40,8 @@ namespace PrivaPub.Infrastructure.Backup
 			[nameof(MaintenanceLock)] = "who is backing up or restoring now",
 			["AppConfiguration"] = "the configuration's copy holds the SMTP password, and is made again from appsettings at boot",
 			[nameof(RestoreRecord)] = "what restores did outlives what they restore",
-			["PersonaArchive"] = "personas' archives being made or imported belong to the moment"
+			["PersonaArchive"] = "personas' archives being made or imported belong to the moment",
+			["ServerSetup"] = "the server is set up once: a restore never opens its setup again"
 		};
 
 		static readonly JsonWriterSettings Json = new() { OutputMode = JsonOutputMode.CanonicalExtendedJson, Indent = false };