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/order_handlers.py

@@ -4,19 +4,19 @@ import logging
 
 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
 
 log = logging.getLogger(__name__)
 
 
-async def on_order_created(event: DomainEvent, session: AsyncSession) -> None:
-    log.info("order.created: order_id=%s", event.payload.get("order_id"))
+async def on_order_created(activity: APActivity, session: AsyncSession) -> None:
+    log.info("order.created: order_id=%s", activity.object_data.get("order_id"))
 
 
-async def on_order_paid(event: DomainEvent, session: AsyncSession) -> None:
-    log.info("order.paid: order_id=%s", event.payload.get("order_id"))
+async def on_order_paid(activity: APActivity, session: AsyncSession) -> None:
+    log.info("order.paid: order_id=%s", activity.object_data.get("order_id"))
 
 
-register_handler("order.created", on_order_created)
-register_handler("order.paid", on_order_paid)
+register_activity_handler("Create", on_order_created, object_type="rose:Order")
+register_activity_handler("rose:OrderPaid", on_order_paid)