feat(journal): federation outbox + push delivery to remote followers

social-federation tasks 5.1–5.6. Completes the inbound-federation
story: a Mastodon follower now receives a trails user's new public
activities in their home timeline.

Outbox (5.1/5.2):
- /users/:username/outbox — paginated OrderedCollection of public
  activities as Create(Note), newest first; unlisted/private never
  federate. Private-user 404 enforced at the route layer because
  Fedify builds collection-level responses from counter/cursors
  without consulting the page dispatcher.
- Note shape: HTML content (escaped name/description/stats + link to
  the activity page) with structured PropertyValue attachments
  (distance-m, elevation-gain-m, duration-s) — Mastodon renders the
  text, trails consumers read the structured fields. Resolves the
  design open question toward Create(Note).
- Authorized Fetch: signed and unsigned outbox fetches deliberately
  see the same (public-only) content until locked accounts exist.

Push delivery (5.3–5.6):
- createActivity / updateActivityVisibility(→public) enqueue one
  deliver-activity job per accepted remote follower; flips away from
  public and hard deletes enqueue Delete(Tombstone) retractions
  (enqueued before the row disappears).
- deliver-activity job: re-reads the row at delivery time (skips if
  gone or no longer public), resolves the recipient inbox via the
  remote_actors cache with actor-document fetch fallback (priming the
  cache), HTTP-signs via the owner's key, and POSTs. retryLimit 8 +
  exponential backoff at enqueue time; outbound paced at 1 req/s per
  remote host.
- Actor objects now advertise the outbox IRI.
- @js-temporal/polyfill added (same range Fedify uses) for published
  timestamps; Fedify's types want the global esnext.temporal namespace,
  bridged with a documented cast.

Tests: 9 unit tests for the AS mapping (escaping, stats, attachments,
published fallback, stable ids, tombstones), 4 outbox integration
tests (collection count, page shape/visibility filtering, private-404,
delivery audience query).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

openspec/changes/social-federation/design.md

@@ -124,6 +124,17 @@ Inbound signature verification uses the actor's public key from their actor obje
   with `/.well-known/trails-cool` as the secondary signal.
 - **Fedify's KvStore is Postgres-backed** (`journal.federation_kv`) so inbox
   replay protection and document caches survive restarts; swept daily.
+- **Outgoing activity shape: `Create(Note)` with PropertyValue attachments**
+  (task 5.1; resolves the open question below toward Mastodon compat). The
+  Note's HTML content carries name/description/stats plus a link to the
+  activity page; distance/elevation/duration ride along as `PropertyValue`
+  attachments that Mastodon ignores gracefully and trails consumers can read
+  without HTML parsing. GPX download links can join once activities have a
+  public GPX endpoint.
+- **Private-user 404 for collection endpoints is enforced at the route layer**
+  (task 5.1): Fedify builds collection-level responses (outbox
+  OrderedCollection) from counter/cursors without consulting the page
+  dispatcher, so the dispatcher's `null` → 404 contract doesn't cover them.
 
 ## Open Questions