feat: replace instance actor decorator with real DB row

Migration inserts a service actor with a well-known UUID. Removes the
InstanceActorUserRepo wrapper — the real ApUserRepository finds it.

crates/adapters/postgres/migrations/022_instance_actor.sql

@@ -0,0 +1,10 @@
+INSERT INTO users (id, username, email, password_hash, display_name, bio)
+VALUES (
+    '00000000-0000-4000-8000-000000000000',
+    'instance',
+    'noreply@instance.invalid',
+    '!service-actor-no-login',
+    NULL,
+    NULL
+)
+ON CONFLICT (id) DO NOTHING;