FEATURE: Automatic translation and localization of posts, topics, categories (#1376)

Related: https://github.com/discourse/discourse-translator/pull/310

This commit includes all the jobs and event hooks to localize posts, topics, and categories.

A few notes:
- `feature_name: "translation"` because the site setting is `ai-translation` and module is `Translation`
- we will switch to proper ai-feature in the near future, and can consider using the persona_user as `localization.localizer_user_id`
- keeping things flat within the module for now as we will be moving to ai-feature soon and have to rearrange
- Settings renamed/introduced are:
  - ai_translation_backfill_rate (0)
  - ai_translation_backfill_limit_to_public_content (true)
  - ai_translation_backfill_max_age_days (5)
  - ai_translation_verbose_logs (false)

plugin.rb

@@ -100,6 +100,7 @@ after_initialize do
     DiscourseAi::Summarization::EntryPoint.new,
     DiscourseAi::AiBot::EntryPoint.new,
     DiscourseAi::AiModeration::EntryPoint.new,
+    DiscourseAi::Translation::EntryPoint.new,
   ].each { |a_module| a_module.inject_into(self) }
 
   register_problem_check ProblemCheck::AiLlmStatus