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Per-document atomic mutation method for the upcoming bulk PATCH endpoint. Tags and receivers merge additively into existing sets; sender and the three location fields replace only when the DTO field is non-blank. Wrapped in its own @Transactional so a per-document failure cannot partially mutate other documents in the outer batch loop. Refs #225 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>