Mastodon clients can sign in: OAuth with a persona per token

OpenIddict 7.7 (MongoDB stores, keys kept in Mongo so tokens survive
restarts) serves /oauth/authorize, /oauth/token, /oauth/revoke and the
discovery documents, including /.well-known/oauth-authorization-server:
- authorization code (PKCE optional) and client credentials, Mastodon's
  scopes including the granular ones, non-expiring reference tokens,
  `created_at` in the token response, and the urn:ietf:wg:oauth:2.0:oob
  page that shows the code;
- /oauth/login signs the private login into a fifteen-minute cookie that
  only /oauth sees (rate limited, antiforgery-protected); /oauth/authorize
  then asks which persona the application acts as. The token's subject
  is that persona's id and nothing else; no root id reaches a token, an
  authorization or a log line;
- the token exchange refuses a persona whose login is banned or deleted,
  and every API request checks the same.

/api/v1/apps registers applications dynamically, /api/v1/apps/
verify_credentials, /api/v1/instance (v1 and v2, "4.2.0 (compatible;
PrivaPub)") and verify_credentials answer in Mastodon's shapes: snake_case
with explicit nulls, Rails-style parameters from query, form or JSON,
{"error": ...} on failure, Link paging. CORS exposes Link.

/api goes to OpenIddict validation and everything else to the existing
JWT; the JWT failure handler no longer sends the exception and stack trace
to the client.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Api/Mastodon/Controllers/AccountsController.cs

@@ -0,0 +1,20 @@
+using Microsoft.AspNetCore.Mvc;
+
+using PrivaPub.Api.Mastodon.Infrastructure;
+using PrivaPub.Api.Mastodon.Mappers;
+
+namespace PrivaPub.Api.Mastodon.Controllers
+{
+	public partial class AccountsController : MastodonController
+	{
+		readonly MastodonMapper _mapper;
+
+		public AccountsController(MastodonMapper mapper)
+		{
+			_mapper = mapper;
+		}
+
+		[HttpGet("/api/v1/accounts/verify_credentials"), Scope("read:accounts")]
+		public async Task<IActionResult> VerifyCredentials(CancellationToken token) => Json(await _mapper.Local(Me, withSource: true, token));
+	}
+}