Implement P05 checked services and safe selected delivery recovery

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a092fe-13b1-7f12-ac74-7d258af4d79c

tests/test_journey_postgres.py

@@ -92,3 +92,16 @@ class PostgresJourneyTests(unittest.TestCase):
         self.assertIsNone(second.user(u.user_id))
         self.seed.commit()
         self.assertEqual(u,second.user(u.user_id))
+
+    def test_two_connections_deliver_one_event_once(self):
+        from user_engine.domain import OutboxEvent
+        event=OutboxEvent(event_id='p05-concurrent',event_type='membership.added',aggregate_id='fixture',tenant=self.tenant,correlation_id='p05',payload={})
+        service=self.service(self.seed)
+        with service.store.transaction():service.store.append_outbox(event)
+        services=[self.service(self.connect()),self.service(self.connect())]
+        barrier=Barrier(2);calls=[]
+        def run(service):
+            barrier.wait(timeout=5)
+            return service.deliver_outbox(self.actor,lambda e:calls.append(e.event_id),worker_id='fixture',event_id=event.event_id)
+        with ThreadPoolExecutor(max_workers=2) as pool:list(pool.map(run,services))
+        self.assertEqual(['p05-concurrent'],calls)