[Fixed] Kombo sync reliability patch

Fixes

• Resolved an issue where backfill jobs could overwrite an existing candidate’s record from another application, causing them to be re-screened and re-contacted.
• Fixed a bug that unintentionally reset a candidate’s AI-opt-out preference during backfill, ensuring opted-out candidates remain unsubscribed.
• Backfill and manual “Process” actions now safely skip applicants who already exist under a different job, keeping candidate timelines and analytics accurate.