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>

README.md

@@ -12,7 +12,7 @@ shared/
   models/                   # Canonical domain models
     user.py                 # User
     magic_link.py           # MagicLink (auth tokens)
-    domain_event.py         # DomainEvent (transactional outbox)
+    domain_event.py         # DomainEvent (legacy — being removed)
     kv.py                   # KeyValue (key-value store)
     menu_item.py            # MenuItem (deprecated — use MenuNode)
     menu_node.py            # MenuNode (navigation tree)
@@ -48,13 +48,13 @@ shared/
     user_loader.py          # Load current user from session
     http_utils.py           # HTTP utility functions
   events/
-    bus.py                  # emit_event(), register_handler()
-    processor.py            # EventProcessor (polls domain_events, runs handlers)
-    handlers/               # Shared event handlers
+    bus.py                  # emit_activity(), register_activity_handler()
+    processor.py            # EventProcessor (polls ap_activities, runs handlers)
+    handlers/               # Shared activity handlers
       container_handlers.py #   Navigation rebuild on attach/detach
       login_handlers.py     #   Cart/entry adoption on login
       order_handlers.py     #   Order lifecycle events
-      ap_delivery_handler.py #  AP activity delivery to follower inboxes
+      ap_delivery_handler.py #  AP activity delivery to follower inboxes (wildcard)
   utils/
     __init__.py
     calendar_helpers.py     # Calendar period/entry utilities
@@ -78,7 +78,7 @@ shared/
 - **App factory:** All apps call `create_base_app()` which sets up DB sessions, CSRF, error handling, event processing, logging, widget registration, and domain service wiring.
 - **Service contracts:** Cross-domain communication via typed Protocols + frozen DTO dataclasses. Apps call `services.calendar.method()`, never import models from other domains.
 - **Service registry:** Typed singleton (`services.blog`, `.calendar`, `.market`, `.cart`, `.federation`). Apps wire their own domain + stubs for others via `register_domain_services()`.
-- **Event bus:** `emit_event()` writes to `domain_events` table in the caller's transaction. `EventProcessor` polls and dispatches to registered handlers.
+- **Activity bus:** `emit_activity()` writes to `ap_activities` table in the caller's transaction. `EventProcessor` polls pending activities and dispatches to registered handlers. Internal events use `visibility="internal"`; federation activities use `visibility="public"` and are delivered to follower inboxes by the wildcard delivery handler.
 - **Widget registry:** Domain services register widgets (nav, card, account); templates consume via `widgets.container_nav`, `widgets.container_cards`.
 - **Cart identity:** `current_cart_identity()` returns `{"user_id": int|None, "session_id": str|None}` from the request session.