Show per-app actor in follow notifications on Hub

Add app_domain to APNotification model and NotificationDTO so follow
notifications display "followed you on blog" instead of just "followed
you" when the follow targets a per-app actor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/alembic/versions/w3u1q9r0s1_add_app_domain_to_notifications.py

@@ -0,0 +1,23 @@
+"""Add app_domain to ap_notifications.
+
+Revision ID: w3u1q9r0s1
+Revises: v2t0p8q9r0
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "w3u1q9r0s1"
+down_revision = "v2t0p8q9r0"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ap_notifications",
+        sa.Column("app_domain", sa.String(30), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ap_notifications", "app_domain")