feat(journal): federation protocol doc + delivery observability

Task group 4 of federation-hardening.

4.1 — FEDERATION.md at the repo root: actor discovery (WebFinger, actor,
NodeInfo), object/activity types with real JSON examples (Note, Create,
Delete, the narrow follow-graph inbox), addressing, HTTP-Signature
expectations, the two-layer dedup contract, durable delivery/retry
policy, and blocklist moderation semantics — precise enough for another
implementation to interoperate. Linked from README and docs/architecture.

4.2 — three prom-client metrics + a journal dashboard row:
- `federation_delivery_total{outcome}` — incremented in deliver-activity
  (delivered/skipped/failed).
- `federation_inbox_dropped_total{reason}` — incremented at every inbox
  drop (duplicate | blocked); this is the counter deferred from task 3.2.
- `federation_queue_depth` — gauge sampled at scrape time in
  /api/metrics from PgBossMessageQueue.getDepth(); the restart-loss
  regression detector.
Grafana journal.json gains a Federation row (delivery rate, queue depth,
inbox drops); the logs panels shift down to make room.

Verified: dashboard JSON valid; journal typecheck + lint clean; unit
suite 357 passing (route-template guard unaffected).

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

docs/architecture.md

@@ -179,6 +179,11 @@ Tech stack:
 
 ## ActivityPub Integration
 
+The concrete wire protocol — actor discovery, object/activity types with
+JSON examples, addressing, signatures, deduplication, delivery retry, and
+moderation — is documented in [FEDERATION.md](../FEDERATION.md) at the repo
+root. This section is the higher-level design intent.
+
 ### Federated Activities
 
 - `Create` Route - Publishing a new route