fix(classes): atomic signup/cancel under per-session advisory lock (D11)

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.

docs/DATABASE_SCHEMA.md

@@ -725,6 +725,7 @@ migration is mirrored as a section in it. Tables/columns added:
 | 042 | Equipment double-booking P0 fix. `CREATE EXTENSION btree_gist` + `equipment_reservations_no_overlap` `EXCLUDE USING gist (equipment_id WITH =, tstzrange(starts_at,ends_at,'[)') WITH &&) WHERE status='reserved'`. Makes the DB the concurrency arbiter (the app-side check-then-insert was a TOCTOU); `reserveEquipment` maps the 23P01 exclusion violation to the friendly overlap message. No new table/column/policy/permission |
 | 043 | `members_update` RLS policy gains a `WITH CHECK` mirroring its `USING` (`user_id = auth.uid()` OR admin/board/treasurer in `space_id`). Defense-in-depth: the post-image is re-validated so a privileged caller cannot move a member row into a space they do not administer. No new table/column |
 | 044 | `prevent_member_self_role_change` extended to also block a non-privileged member changing their own `payment_status`, `payment_note`, `dues_paid_until`, `last_paid_at`, `last_payment_at`, `stripe_customer_id`, `joined_at` (RLS lets a member UPDATE their own row, so these were self-settable to forge a dues-good signal). Service-client (`auth.uid()` NULL) and privileged-on-another-member writes still bypass. No new table/column |
+| 045 | `class_signup_tx(p_session_id,p_space_id,p_member_id)` + `class_cancel_tx(...)` SECURITY DEFINER, `search_path=public`. Each takes `pg_advisory_xact_lock(hashtext(session_id))` then does the capacity-decision+dup-check+insert (signup) or cancel+conditional waitlist-promote (cancel) atomically. Closes the concurrent over-enroll / double-promote races (dynamic capacity can't be a constraint). Mirrors `computeSignupStatus`/`pickPromotion`; the SQL is the runtime authority. No new table/column/policy |
 
 New enum: `comment_entity_type` = `forum_thread | proposal | incident | policy`.