docs(run-ops db): document per-table divergence from the control-plane schema

Rework the run-ops schema comments so they read as documentation rather than
working notes. Replaces the top-of-file header with prose describing the
subset relationship and the three relation-handling rules, adds a short header
above each model stating how it differs from its control-plane counterpart
(which relations are scalar-only FKs, which are FK-free join models, which stay
as real FKs), and removes the stale group-(A)/group-(B)/"drop" tags. Comments
only; no model, field, type, attribute, index, or enum is changed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: d-cs