feat(notifications+admin): marketing campaigns

- campaigns table (migration 19) + CRUD + send endpoint in notification-svc
- audience resolution reads cross-schema from identity.users (all / verified /
  with_plan); send dispatches via the SMS or Email channel and logs deliveries
- endpoints: GET/POST /v1/campaigns, POST /v1/campaigns/:id/send, DELETE
- gateway route /v1/campaigns/* → notification
- /admin/marketing: create campaign (channel, audience, template/subject/body),
  list with status + sent counts, send, delete

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/gateway/cmd/server/main.go

@@ -142,6 +142,7 @@ func main() {
 	v1.Any("/channels/*path",               apiRL, auth, notification.Handler())
 	v1.Any("/sms/*path",                    apiRL, auth, notification.Handler())
 	v1.Any("/email/*path",                  apiRL, auth, notification.Handler())
+	v1.Any("/campaigns/*path",              apiRL, auth, notification.Handler())
 
 	log.Printf("gateway listening on :%s", port)
 	if err := r.Run(":" + port); err != nil {