FIX: Ensure untranslated post ids exclude those already translated to target locale

[nattsw]

This PR corrects the logic in fetch_untranslated_model_ids to prevent posts/topics that already have a translation in the target locale from being returned for backfill. Previously, the SQL query only excluded posts where the detected locale and translation were both present for the target locale, resulting in posts with existing translations being included when their detected_locale was different.

Related: #255

[Copilot]

Pull Request Overview

This PR fixes the logic in fetch_untranslated_model_ids so that posts with an existing translation in the target locale are correctly excluded, even if their detected locale differs.

• Added a spec to validate that posts with target translations are not returned.
• Simplified the SQL query in fetch_untranslated_model_ids to use two separate NOT IN clauses for excluding posts with a detected target locale or a translation in the target locale.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
spec/jobs/automatic_translation_backfill_spec.rb	Added a new test ensuring posts with a target locale translation are excluded.
app/jobs/scheduled/automatic_translation_backfill.rb	Refactored the SQL query to properly exclude posts with an existing target translation.