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>

models/__init__.py

@@ -8,8 +8,6 @@ from .ghost_membership_entities import (
     GhostNewsletter, UserNewsletter,
     GhostTier, GhostSubscription,
 )
-from .domain_event import DomainEvent
-
 from .ghost_content import Tag, Post, Author, PostAuthor, PostTag, PostLike
 from .page_config import PageConfig
 from .order import Order, OrderItem