Unify domain_events + ap_activities into AP-shaped event bus

All cross-service events now flow through ap_activities with a unified
EventProcessor. Internal events use visibility="internal"; federation
activities use visibility="public" and get delivered by a wildcard handler.

- Add processing columns to APActivity (process_state, actor_uri, etc.)
- New emit_activity() / register_activity_handler() API
- EventProcessor polls ap_activities instead of domain_events
- Rewrite all handlers to accept APActivity
- Migrate all 7 emit_event call sites to emit_activity
- publish_activity() sets process_state=pending directly (no emit_event bridge)
- Migration to drop domain_events table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/federation_publish.py

@@ -1,9 +1,9 @@
 """Inline federation publication — called at write time, not via async handler.
 
-Replaces the old pattern where emit_event("post.published") → async handler →
-publish_activity().  Now the originating service calls try_publish() directly,
-which creates the APActivity in the same DB transaction.  AP delivery
-(federation.activity_created → inbox POST) stays async.
+The originating service calls try_publish() directly, which creates the
+APActivity (with process_state='pending') in the same DB transaction.
+The EventProcessor picks it up and the delivery wildcard handler POSTs
+to follower inboxes.
 """
 from __future__ import annotations