feat(ntfy): dispatch gateway_online/gateway_offline from model

app/models/gateway.rb

@@ -1,4 +1,6 @@
 class Gateway < ApplicationRecord
+  include NtfyDispatchable
+
   # Normalize metadata to always be a Hash
   attribute :metadata, :jsonb, default: {}
 
@@ -35,12 +37,31 @@ class Gateway < ApplicationRecord
 
   # Update heartbeat timestamp and status
   def heartbeat!
+    was_offline = status == "offline"
     update!(status: "online", last_heartbeat_at: Time.current)
+
+    if was_offline
+      self.class.dispatch_ntfy("gateway_online",
+        title: "Gateway online",
+        message: "#{name} (#{device_id}) came back online",
+        priority: 3,
+        tags: ["white_check_mark"],
+        click: nil)
+    end
   end
 
   # Mark gateway as offline
   def mark_offline!
+    was_online = status == "online"
     update!(status: "offline")
+
+    if was_online
+      self.class.dispatch_ntfy("gateway_offline",
+        title: "Gateway offline",
+        message: "#{name} (#{device_id}) went offline",
+        priority: 4,
+        tags: ["rotating_light"])
+    end
   end
 
   # Increment message counters