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>

events/__init__.py

@@ -1,4 +1,9 @@
-from .bus import emit_event, register_handler
+from .bus import emit_activity, register_activity_handler, get_activity_handlers
 from .processor import EventProcessor
 
-__all__ = ["emit_event", "register_handler", "EventProcessor"]
+__all__ = [
+    "emit_activity",
+    "register_activity_handler",
+    "get_activity_handlers",
+    "EventProcessor",
+]