Everything on, phase 4b: ids that resolve, a hashtag page, grouped notifications, remote accounts' real counts

- A community's announces resolve, as FEDERATION.md and ROADMAP always said Announce ids do. GroupDistributor keeps
  each one it sends (GroupAnnouncement, unique by id). /grunts serves it while the post it carries is shown and 410
  after, and also serves the announce-{postId} ids the group outbox lists, which now keep a stable `published`
  instead of the time of the fetch.
- A persona's boost points at the boosted post: its `url` in the Mastodon API is the boosted post's page, and a browser
  following the boost's id is redirected there instead of getting JSON.
- /tags/{tag}, where every Hashtag link we send points, is now a public page of this server's public posts with that
  tag, with the same strict CSP and noindex as the profile pages.
- Grouped notifications (/api/v2/notifications, its unread count, a group, its accounts and dismiss). We advertise
  api_versions.mastodon = 7 so clients show quotes, and clients that trust it call these; they answered 404. Likes and
  boosts of one post group together, as do follows within an hour. The version string stays 4.2.0 until streaming
  and Web Push exist.
- A remote account's follower, following and post counts are what its server publishes. AccountCountsJob reads its
  collections' totalItems from its own origin, signed by the instance actor, at most daily and only after the account
  was fetched, never when someone looks. They used to be 0.
- The other ids that do not resolve are documented as such: Update, Delete, EmojiReact, QuoteRequest and its answers,
  Flag, Ignore and poll votes.

676 tests pass.

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

FEDERATION.md

@@ -204,9 +204,12 @@ Posts with a location (shown to nearby users of this server) never leave the ser
 - **HTML.** Received HTML is sanitised to Mastodon's allowlist.
 - **Keys we cannot fetch for now.** When a sender's key cannot be fetched because its server timed out or answered
   5xx, the inbox answers 503 with `Retry-After: 300` rather than 401.
-- **Activity ids.** `Create` and `Announce` ids dereference. `Follow`, `Like`, `Block` and the `Accept`, `Reject` and
-  `Undo` that answer them do not, because serving them would reveal who follows, likes and blocks whom; they are always
-  sent with their object embedded.
+- **Activity ids.** `Create` and `Announce` ids dereference, a community's announces included, until the post they
+  carry is deleted (then 410); a persona's boost id sends a browser to the boosted post. `Follow`, `Like`, `Block` and
+  the `Accept`, `Reject` and `Undo` that answer them do not, because serving them would reveal who follows, likes and
+  blocks whom. Neither do `Update`, `Delete`, `EmojiReact`, `QuoteRequest` and its answers, `Flag`, `Ignore` or poll
+  votes. All of them are always sent with their object embedded.
+- **Hashtags.** A post's `Hashtag` links go to `/tags/{tag}`, a public page of this server's public posts with that tag.
 - **Delivery.** Failed deliveries are retried with Mastodon's backoff (16 attempts). A host that keeps failing is paused,
   starting at an hour and growing to a week.