[B2C][WAITLIST] Slot assignment runs outside the checkout transaction; partial multi-session enrollment on mid-loop failure

[teetangh]

Problem

lib/waitlist/slot-handler.ts assigns freed slots outside the booking tx, and multi-session class enrollment can fail mid-loop leaving partial enrollment. No unique constraint on the SlotOfAppointment↔User m:n protects against double-assignment.

Fix shape

Move assignment into the same tx as the slot release (or CAS-claim per slot); add the missing unique constraint at the pre-MVP schema reset; make multi-session enrollment all-or-nothing.

[teetangh]

Triage 2026-08-01 (from #1072 §6.11) — premise file no longer exists

This issue is written against lib/waitlist/slot-handler.ts, which assigned freed slots outside the booking transaction.

Verified against dev at 839d359c: that file is gone. lib/waitlist/ now contains only service.ts and tokens.ts, following PR #1035 (feat(waitlist): retire the event queue, rebuild Waitlist as the newsletter), which repurposed the Waitlist model entirely and deleted the event-queue behaviour this issue describes.

That does not automatically mean the concern is resolved. The two underlying risks — slot assignment happening outside the checkout transaction, and multi-session class enrolment failing mid-loop into a partial state — may have been designed away with the queue, or may have moved somewhere else in the booking path. The missing unique constraint on the SlotOfAppointment↔User join is a separate, still-checkable question that the rework would not have touched.

Recommended action: re-verify against the current booking and enrolment path before doing anything else, then either restate this issue against the code that exists now or close it. Left launch: pre-mvp under the #837 umbrella until that check happens, because assuming it is fine is the more expensive error.

[teetangh]

Closing as obsolete (via PR #1091).

Premise file lib/waitlist/slot-handler.ts no longer exists. Event capacity waitlists were retired in #1035; Waitlist is now the marketing newsletter (lib/waitlist/service.ts + tokens.ts only). Concurrent enrollment uniqueness remains a schema/allocator concern, not a waitlist-assignment path.