Records the current recommendation (stay on Mongo + targeted hardening),
the realistic full-migration cost (3.5–6 months), and the trigger
conditions under which we should revisit the decision. Prompted by the
multi-seller orphan-payment bug on 2026-05-28 — exactly the FK-shaped
class of bug Postgres would prevent, but not by itself worth a migration.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
