Deferred deeper-audit P1. signUpForClass (count->decide->insert) and
cancelMySignup (cancel->recount->promote) were non-atomic: concurrent
signups at the capacity boundary over-enrolled and concurrent cancels
double-promoted waitlisters (dynamic capacity can't be a DB constraint).
Migration 045 adds class_signup_tx/class_cancel_tx (SECURITY DEFINER,
pg_advisory_xact_lock per session) doing the decision+write atomically;
actions call the RPCs and keep all pre-checks. Removed the now-unused
effectiveCapacity/computeSignupStatus/pickPromotion imports +
isUniqueViolation (pure rules still live + tested in classes-logic).
528 tests; build clean.
