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/Models/ServerSetup.cs

@@ -0,0 +1,64 @@
+using MongoDB.Driver;
+using MongoDB.Entities;
+
+namespace PrivaPub.Models
+{
+	// The server's first-time setup (owner decision 2026-10-16): while no administrator exists, the client's setup page makes
+	// the first login, which becomes the administrator; this document then closes setup for good, whoever is demoted or
+	// deleted later (the CLI stays the way back). The first request to insert it wins, so two setups at once make one
+	// administrator. A backup never holds it and a restore never drops it (ServerBackup.Excluded): a restore tombstones an
+	// administrator made after the backup, and setup would otherwise be open to whoever came first.
+	public class ServerSetup : Entity
+	{
+		public const string Name = "setup";
+
+		public string AdminId { get; set; }//null while the claiming request is still making its login
+		public DateTime SealedAt { get; set; }
+
+		public static async Task<bool> IsPending(CancellationToken token) =>
+			!await DB.Default.Find<ServerSetup>().Match(s => s.ID == Name).ExecuteAnyAsync(token);
+
+		/// <summary>Takes the setup for one request: false when it is done, or another request took it first.</summary>
+		public static async Task<bool> Claim(CancellationToken token)
+		{
+			try
+			{
+				await DB.Default.InsertAsync(new ServerSetup { ID = Name, SealedAt = DateTime.UtcNow }, token);
+				return true;
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				return false;
+			}
+		}
+
+		/// <summary>Names the administrator the claiming request made.</summary>
+		public static Task Claimed(string adminId, CancellationToken token) =>
+			DB.Default.Update<ServerSetup>().Match(s => s.ID == Name && s.AdminId == null).Modify(s => s.AdminId, adminId).ExecuteAsync(token);
+
+		/// <summary>Gives a claim back when its login couldn't be made; a named administrator is never given back.</summary>
+		public static Task Release() =>
+			DB.Default.DeleteAsync<ServerSetup>(s => s.ID == Name && s.AdminId == null);//a delete takes no token outside a transaction
+
+		/// <summary>Closes setup for an administrator made elsewhere (the CLI, a server that had one already).</summary>
+		public static async Task Seal(string adminId, CancellationToken token)
+		{
+			try
+			{
+				await DB.Default.Update<ServerSetup>().Match(s => s.ID == Name)
+					.Modify(b => b.SetOnInsert(s => s.AdminId, adminId))
+					.Modify(b => b.SetOnInsert(s => s.SealedAt, DateTime.UtcNow))
+					.Option(o => o.IsUpsert = true)
+					.ExecuteAsync(token);
+			}
+			catch (MongoCommandException ex) when (ex.Code == 11000)
+			{
+				//sealed meanwhile
+			}
+			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
+			{
+				//sealed meanwhile
+			}
+		}
+	}
+}