One sign-in for decePubClient: the root JWT exchanged for a persona's token

The first-party client signs in on /clientapi and exchanges that JWT on
/oauth/token (RFC 8693, subject_token_type jwt, avatar_id) for one
persona's Mastodon token. Only the seeded public application `decepub`
holds the grant; RootJwtSubjectToken validates the JWT through RootJwt,
which JwtBearer now shares (signature, lifetime, ban, deletion, session
stamp). The issued token names the avatar, never the root.

Owner decision recorded in ROADMAP; it supersedes "moving decePubClient
onto the Mastodon API is out of scope".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Program.cs

@@ -1,4 +1,4 @@
-using Microsoft.AspNetCore.HttpOverrides;
+using Microsoft.AspNetCore.HttpOverrides;
 using Microsoft.Extensions.Options;
 
 using MongoDB.Bson;
@@ -186,6 +186,7 @@ try
 		var passwordHasher = app.Services.GetService(typeof(IPasswordHasher)) as IPasswordHasher;
 		await dbClient.Init(passwordHasher);
 		await app.Services.GetRequiredService<PrivaPub.Federation.Moderation.IDomainBlocks>().Reload(CancellationToken.None);
+		await PrivaPub.Api.Mastodon.Auth.PersonaExchange.EnsureFirstPartyClient(app.Services, CancellationToken.None);
 	}
 	catch (Exception ex)
 	{