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>

services/federation_impl.py

@@ -183,16 +183,21 @@ class SqlFederationService:
 
         now = datetime.now(timezone.utc)
 
+        actor_url = f"https://{domain}/users/{username}"
+
         activity = APActivity(
             activity_id=activity_uri,
             activity_type=activity_type,
             actor_profile_id=actor.id,
+            actor_uri=actor_url,
             object_type=object_type,
             object_data=object_data,
             published=now,
             is_local=True,
             source_type=source_type,
             source_id=source_id,
+            visibility="public",
+            process_state="pending",
         )
         session.add(activity)
         await session.flush()
@@ -208,7 +213,7 @@ class SqlFederationService:
                     ],
                     "id": activity_uri,
                     "type": activity_type,
-                    "actor": f"https://{domain}/users/{username}",
+                    "actor": actor_url,
                     "published": now.isoformat(),
                     "object": {
                         "type": object_type,
@@ -221,21 +226,6 @@ class SqlFederationService:
         except Exception:
             pass  # IPFS failure is non-fatal
 
-        # Emit domain event for downstream processing (delivery)
-        from shared.events import emit_event
-        await emit_event(
-            session,
-            "federation.activity_created",
-            "APActivity",
-            activity.id,
-            {
-                "activity_id": activity.activity_id,
-                "activity_type": activity_type,
-                "actor_username": username,
-                "object_type": object_type,
-            },
-        )
-
         return _activity_to_dto(activity)
 
     # -- Queries --------------------------------------------------------------