PERF: fixes N+1 for automation plugin (#32810)

- Add a safeguard until we implement pagination for the index, it
ensures we won't try to load more than 500 automations ever.
- Includes the fields of the automation so we don't have an N+1 and also
filter them in ruby land instead of doing two additional queries
- Ensures a field object in the field serializer is capable to find the
loaded automation associated to it instead of trying to reload it
- Includes pending_automations to avoid an N+1

plugins/automation/app/models/discourse_automation/automation.rb

@@ -7,7 +7,8 @@ module DiscourseAutomation
     has_many :fields,
              class_name: "DiscourseAutomation::Field",
              dependent: :delete_all,
-             foreign_key: "automation_id"
+             foreign_key: "automation_id",
+             inverse_of: :automation
     has_many :pending_automations,
              class_name: "DiscourseAutomation::PendingAutomation",
              dependent: :delete_all,
@@ -28,6 +29,16 @@ module DiscourseAutomation
 
     attr_accessor :running_in_background
 
+    def trigger=(new_trigger)
+      @triggerable = nil
+      super
+    end
+
+    def script=(new_script)
+      @scriptable = nil
+      super
+    end
+
     def running_in_background!
       @running_in_background = true
     end