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/sqlite/migrations/0022_instance_actor.sql

@@ -0,0 +1,8 @@
+INSERT OR IGNORE INTO users (id, email, password_hash, created_at, username)
+VALUES (
+    '00000000-0000-4000-8000-000000000000',
+    'noreply@instance.invalid',
+    '!service-actor-no-login',
+    datetime('now'),
+    'instance'
+);