T7: the federation surface over HTTP

Tests through the real routes of PeasantsController, WellKnownController and
UsersController, on the whole server under test (34 new tests):

- FederationGetTests: the actor document (activity+json, SPKI key at
  #main-key owned by the actor, sharedInbox, published = PublishedOn's day,
  no creation date, no root); ld+json; browsers sent to /@name; Vary: Accept;
  /users 301; the outbox's totalItems and ?page=true&max_id paging across
  the 20-item boundary, boosts as Announces, and no followers-only, direct,
  located, federated-copy or deleted post; /groupies and /stalking naming
  nobody; /trophies (public pins, newest first) and /tattoos; /scribbles
  (public and unlisted 200, browsers redirected, followers-only, direct and
  located 404, deleted 410 Tombstone); a circle post only for a signed member
  or its instance actor; a circle's /groupies, /flock and /wardens only for
  members; /grunts create- and announce- ids; /parrot-licences 200, revoked
  410, wrong author 404; secure mode's 401 for every unsigned GET but the
  instance actor's.
- WellKnownTests: WebFinger by acct:, @-prefixed, bare, upper-case and actor
  URI; other domains, unknown names, a root's login name and no resource;
  the instance actor, a community, a circle (answered: current behaviour);
  NodeInfo links, 2.0 and 2.1 naming no root, unknown versions 404; usage
  counting only public, unlisted, non-boost local posts (Exclusive).
- InboxRouteTests: all three inboxes accept a signed delivery and refuse
  junk (400), unsigned (401), a bad Digest, a two-hour-old Date, a signature
  for another host and a swapped body (401); an unknown persona's /mouth is
  404; ld+json with the ActivityStreams profile is accepted; a signer whose
  actor answers 503 gets 503 with Retry-After; the 301st unsigned POST from
  one address is 429 while a signed server from it is not.
- PersonaSeparationHttpTests: with a sibling persona and its community on
  the same login, every GET under /api (filled with the persona's ids) plus
  search, lookup and relationships, and a crawl of everything federation
  publishes about the persona (actor, outbox pages, collections, scribbles,
  grunts, WebFinger, NodeInfo, /@ pages), never name the sibling, its
  community or the login.

Fixed:
- A circle's /groupies told anyone how many followers (members) it has,
  while its /flock and /wardens were already for members only; it now
  answers 404 to anyone but a signed member or a member's instance actor.
- WebFinger answered 404 to a bare user@domain or @user@domain resource,
  which Mastodon, GoToSocial and Pleroma all accept; it now treats them as
  acct: (noted in docs/INTEROP.md).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

docs/INTEROP.md

@@ -26,7 +26,8 @@ Priorities, used throughout:
   - Served documents are `application/activity+json; charset=utf-8`, which is on Lemmy's exact allowlist and is
     accepted by GoToSocial and Misskey.
   - WebFinger answers `application/jrd+json`, and answers for the actor URL as well as `acct:`. Akkoma and
-    Iceshrimp.NET require both.
+    Iceshrimp.NET require both. A bare `user@domain` or `@user@domain` is taken as an `acct:` too, as Mastodon,
+    GoToSocial and Pleroma take it (2026-10-03).
 - **SSRF guard:**
   - Its IPv6 rule is an allowlist (`2000::/3` only), so IPv4-compatible `::a.b.c.d`, NAT64 and the other tricks behind
     Mastodon's GHSA-vwhj (Jul 2026) are already refused.