feat: replace instance actor decorator with real DB row

Migrations (postgres + sqlite) insert a service actor with a well-known
UUID. Removes the InstanceActorUserRepo wrapper.

crates/adapters/postgres/migrations/0022_instance_actor.sql

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