DEV: Unhide spam detection setting (#1374)

## 🔍 Overview
We want to unhide `ai_spam_detection_enabled` setting so that we can retain staff action log features. However, we also want to ensure users cannot enable spam detection without having `AiModerationSetting.spam` present in the database.

In this update we unhide the setting, but also add a validator to ensure the necessary configuration is in place before allowing the setting to be enabled.

config/locales/server.en.yml

@@ -56,6 +56,8 @@ en:
     ai_nsfw_flag_threshold_sexy: "Threshold for an image classified as sexy to be considered NSFW."
     ai_nsfw_models: "Models to use for NSFW inference."
 
+    ai_spam_detection_enabled: "Enable the AI spam detection module"
+
     ai_openai_api_key: "API key for OpenAI API. ONLY used for Image creation and edits. For GPT use the LLM config tab"
     ai_openai_image_generation_url: "URL for OpenAI image generation API"
     ai_openai_image_edit_url: "URL for OpenAI image edit API"
@@ -275,6 +277,7 @@ en:
       invalid_error_type: "Invalid error type provided"
       unexpected: "An unexpected error occured"
       bot_user_update_failed: "Failed to update the spam scanning bot user"
+      configuration_missing: "The AI spam detection configuration is missing. Add configuration in the 'Admin > Plugins > Discourse AI > Spam' before enabling."
 
     ai_bot:
       reply_error: "Sorry, it looks like our system encountered an unexpected issue while trying to reply.\n\n[details='Error details']\n%{details}\n[/details]"

config/settings.yml

@@ -374,7 +374,7 @@ discourse_ai:
 
   ai_spam_detection_enabled:
     default: false
-    hidden: true
+    validator: "DiscourseAi::Configuration::SpamDetectionValidator"
   ai_spam_detection_user_id:
     default: ""
     hidden: true

lib/configuration/spam_detection_validator.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module DiscourseAi
+  module Configuration
+    class SpamDetectionValidator
+      def initialize(opts = {})
+        @opts = opts
+      end
+
+      def valid_value?(val)
+        return true if Rails.env.test?
+        return true if AiModerationSetting.spam
+
+        false
+      end
+
+      def error_message
+        I18n.t("discourse_ai.spam_detection.configuration_missing")
+      end
+    end
+  end
+end