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/handlers/container_handlers.py

@@ -2,18 +2,18 @@ from __future__ import annotations
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from shared.events import register_handler
-from shared.models.domain_event import DomainEvent
+from shared.events import register_activity_handler
+from shared.models.federation import APActivity
 from shared.services.navigation import rebuild_navigation
 
 
-async def on_child_attached(event: DomainEvent, session: AsyncSession) -> None:
+async def on_child_attached(activity: APActivity, session: AsyncSession) -> None:
     await rebuild_navigation(session)
 
 
-async def on_child_detached(event: DomainEvent, session: AsyncSession) -> None:
+async def on_child_detached(activity: APActivity, session: AsyncSession) -> None:
     await rebuild_navigation(session)
 
 
-register_handler("container.child_attached", on_child_attached)
-register_handler("container.child_detached", on_child_detached)
+register_activity_handler("Add", on_child_attached, object_type="rose:ContainerRelation")
+register_activity_handler("Remove", on_child_detached, object_type="rose:ContainerRelation")