feat(ntfy): dispatch gateway_offline from CheckGatewayHealthJob after bulk update

app/jobs/check_gateway_health_job.rb

@@ -2,13 +2,24 @@ class CheckGatewayHealthJob < ApplicationJob
   queue_as :default
 
   def perform
-    # Mark gateways as offline if no heartbeat in last 2 minutes
-    offline_count = Gateway.where("last_heartbeat_at < ?", 2.minutes.ago)
-                          .where.not(status: "offline")
-                          .update_all(status: "offline")
+    # Capture stale gateway details BEFORE update_all (update_all bypasses callbacks
+    # and would change the scope, so we snapshot the rows first)
+    stale_gateway_details = Gateway.where("last_heartbeat_at < ?", 2.minutes.ago)
+                                   .where.not(status: "offline")
+                                   .pluck(:id, :name, :device_id)
+
+    offline_count = Gateway.where(id: stale_gateway_details.map(&:first)).update_all(status: "offline")
 
     if offline_count > 0
       Rails.logger.warn("Marked #{offline_count} gateways as offline due to missing heartbeat")
+
+      stale_gateway_details.each do |_id, name, device_id|
+        Gateway.dispatch_ntfy("gateway_offline",
+          title: "Gateway offline",
+          message: "#{name} (#{device_id}) went offline — no heartbeat for 2+ minutes",
+          priority: 4,
+          tags: ["rotating_light"])
+      end
     end
   end
 end