feat(push-gateway,msg): redis pubsub backplane for real-time notifications

M7: ADR-040 Redis Pub/Sub as state routing backplane

- push-gateway: remove Kafka consumer, add SSE endpoint

- SSE: subscribe to Redis user:{userId}:notify on connect

- msg: publish notifications to Redis Pub/Sub instead of HTTP push

- docker-compose: remove Kafka env from push-gateway

services/push-gateway/internal/redisclient/pubsub.go

@@ -359,6 +359,16 @@ func idempotentKey(eventID string) string {
 	return "edu:push:idempotent:" + eventID
 }
 
+// NotifyChannel returns the Redis Pub/Sub channel name for a user's real-time
+// notification stream. M7 (ADR-040): msg service publishes notifications to
+// this channel; realtime-gateway instances subscribe per-connection when a
+// user connects via SSE.
+//
+// Channel naming: `user:{userId}:notify`
+func NotifyChannel(userID string) string {
+	return "user:" + userID + ":notify"
+}
+
 // ParseUserChannel extracts the userID from a "edu:push:channel:user:<userID>"
 // channel name. Returns empty string when the name does not match.
 func ParseUserChannel(channel string) string {