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>