booking event fan-out lets one side effect block the rest

[Hqzdev]

Problem

Booking event fan-out is not isolated. One failed side effect can prevent later side effects from running, even when they are independent.

Evidence

• packages/api/src/shared/events/booking-events.ts:32 emits all registered handlers through Promise.all and rejects the whole emit when any handler rejects.
• packages/api/src/shared/events/register-booking-event-handlers.ts:20 registers a booking-created handler that performs multiple side effects sequentially.
• packages/api/src/shared/events/register-booking-event-handlers.ts:23 awaits calendar sync enqueue before reminder enqueue.
• packages/api/src/shared/events/register-booking-event-handlers.ts:30 only enqueues the reminder after calendar sync succeeds.
• packages/api/src/shared/events/register-booking-event-handlers.ts:35 and :46 have the same pattern for cancellation/update fan-out.

Impact

A temporary calendar queue failure can skip booking reminders. A WebSocket broadcast failure can prevent queue jobs from being created. This creates partial side effects with no DLQ or retry record for the skipped work.

Expected Fix

Make event side effects independent and observable:

• run fan-out tasks with Promise.allSettled or separate handlers;
• log each failed side effect with event name, booking id, and dependency name;
• send retryable side effects to a queue/DLQ instead of skipping them;
• add tests that one failing side effect does not stop the others.