feat: notification service

gateway/docs/examples.md

@@ -173,9 +173,9 @@ User-wide event:
 ```bash
 redis-cli XADD gateway:client-events '*' \
   user_id user-123 \
-  event_type fleet.updated \
-  event_id event-123 \
-  payload_bytes payload-v1
+  event_type game.turn.ready \
+  event_id notification-route-123 \
+  payload_bytes flatbuffers-game-turn-ready
 ```
 
 Session-targeted event with correlation:
@@ -194,6 +194,8 @@ redis-cli XADD gateway:client-events '*' \
 Notes:
 
 - `payload_bytes` in Redis Stream entries must be binary-safe payload data;
+- notification-owned payload bytes should follow
+  `pkg/schema/fbs/notification.fbs`;
 - the gateway derives `timestamp_ms`, recomputes `payload_hash`, and signs the
   outgoing event at delivery time;
 - each gateway replica consumes streams with plain `XREAD`, so publishers must