Fediverse: federate reposts as Announce

[wintermeyer]

TL;DR A repost (boost) by a federating member is invisible to their Mastodon followers — repost_post/2 has no federation hook. Emit an Announce on repost and Undo(Announce) on un-repost. Split out of #843 (Part 1); parent tracker #784.

Where: Vutuv.Posts.repost_post/2 + unrepost_post/2 (lib/vutuv/posts.ex, ~line 647); activity builders in VutuvWeb.Fediverse.Docs; enqueue + gates in Vutuv.Fediverse.maybe_federate/2. Architecture: docs/architecture/fediverse.md.

Now: repost_post/2 inserts the PostRepost row and only broadcasts the in-app feed event (broadcast_new_repost/1); unrepost_post/2 deletes the row. Neither federates — unlike create_post/update_post/delete_post, which call Vutuv.Fediverse.federate_*. So a boost never leaves vutuv.

Want:

• Reposting enqueues an Announce to the reposter's Fediverse followers, but only when all three hold: (1) the reposter federates (Vutuv.Fediverse.federated?/1), (2) the reposted post is fully public (Vutuv.Posts.restricted?/1 false — already guaranteed at repost time by check_visibility_lock/2), (3) the original author federates too — the Announce object is the original Note's id (https://<host>/<author>/posts/<id>); for a non-federating author that URL serves no Note, so Mastodon could not render the boost.
• Un-reposting enqueues the matching Undo(Announce).
• Activity ids must be stable so the Undo references the same Announce (e.g. <note-url>#announce-<reposter-username>).

 reposter's followers' servers
        ^
        |  Announce { object: https://<host>/<original-author>/posts/<id> }
        |
  repost_post/2   -> Vutuv.Fediverse.federate_repost(post, reposter)
  unrepost_post/2 -> Vutuv.Fediverse.federate_unrepost(post, reposter)

Notes for someone new to the code:

• Hook inside repost_post/2 where both the %PostRepost{} and the original %Post{} (with post.user_id) are in scope — broadcast_new_repost/1 alone doesn't know the original author.
• Follow the shape of Vutuv.Fediverse.maybe_federate/2 for the federated?/public gates and the delivery-queue enqueue.
• Tests: test/vutuv/posts_federation_test.exs covers the existing Create/Update/Delete hooks — use it as the template. Start with a test (project rule).

Acceptance:

• Repost by a federating member of a federating member's public post → one Announce per distinct follower inbox; un-repost → Undo(Announce) with a matching id.
• Repost of a non-federating author's post, or by a non-federating member → nothing enqueued.
• mix precommit green; any new gettext strings ship with German translations.