feat(timeslots): notify customers when a booked timeslot is deleted

[choden-dev]

Summary

Investigated whether Payload notifies customers when a pickup timeslot is deleted. It did not: the Timeslots collection only had an afterChange hook (for edits), with no afterDelete. Deleting a booked timeslot left affected orders with a dangling reference and gave customers no notice.

This PR adds customer notification on timeslot deletion.

Changes

• lib/templates/timeslotDeleted.pug (new): apology email template that reassures the customer their order is unaffected and asks them to select a new pickup slot. Uses the existing shared mixins/styles. No em dashes in the copy.
• lib/email.ts: new sendTimeslotDeletedEmail(), following the existing sendTimeslotChangedEmail pattern. Subject: Action Needed: Select a New Pickup Time — <orderNumber>.
• collections/Timeslots.ts: new afterDelete hook notifyOnTimeslotDelete that:
  • finds all orders referencing the deleted timeslot,
  • clears each order's pickupTimeslot reference so the customer can re-select,
  • emails each affected customer (fire-and-forget so the admin UI is not blocked).

This mirrors the existing afterChange notification behavior and complements the existing Orders afterDelete bookedCount handling.

Testing

• tsc --noEmit passes for changed files.
• biome check clean.