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>

alembic/versions/n4l2i8j0k1_drop_domain_events_table.py

@@ -0,0 +1,46 @@
+"""drop domain_events table
+
+Revision ID: n4l2i8j0k1
+Revises: m3k1h7i9j0
+Create Date: 2026-02-22
+
+The domain_events table is no longer used — all events now flow through
+ap_activities with the unified activity bus.
+"""
+
+revision = "n4l2i8j0k1"
+down_revision = "m3k1h7i9j0"
+branch_labels = None
+depends_on = None
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSONB
+
+
+def upgrade() -> None:
+    op.drop_index("ix_domain_events_state", table_name="domain_events")
+    op.drop_index("ix_domain_events_event_type", table_name="domain_events")
+    op.drop_table("domain_events")
+
+
+def downgrade() -> None:
+    op.create_table(
+        "domain_events",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+        sa.Column("event_type", sa.String(128), nullable=False),
+        sa.Column("aggregate_type", sa.String(64), nullable=False),
+        sa.Column("aggregate_id", sa.Integer(), nullable=False),
+        sa.Column("payload", JSONB(), nullable=True),
+        sa.Column("state", sa.String(20), nullable=False, server_default="pending"),
+        sa.Column("attempts", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("max_attempts", sa.Integer(), nullable=False, server_default="5"),
+        sa.Column("last_error", sa.Text(), nullable=True),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True),
+            nullable=False, server_default=sa.func.now(),
+        ),
+        sa.Column("processed_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index("ix_domain_events_event_type", "domain_events", ["event_type"])
+    op.create_index("ix_domain_events_state", "domain_events", ["state"])