Feature/82 make coach selection mandatory for private lessons

[maxiDeee]

Overview

Private lessons could be created without a coach (the form validated coach_id but never saved it, and the column was nullable). This makes a coach mandatory for private lessons end to end, lets admins reassign the coach when editing, and prevents deleting a coach who is still assigned to a private lesson.

• Schema: coach_id FK set null → restrict, plus a partial CHECK (type <> 'private_lessons' OR coach_id IS NOT NULL) so private lessons require a coach while programs stay coachless.
• Create/edit: createService now persists coachId; updateService supports reassignment while keeping the coach required. The dialog submits the coach via a hidden input (a Radix Select with name silently falls back to its first option) and pre-selects the current coach when editing.
• Delete: new deleteUserAdmin action blocks deleting a coach assigned to a private lesson; surfaced in the Users tab via a delete button, confirm dialog, and error toast.
• Also fixes a pre-existing bug that blocked reassignment: updateService recreated the Stripe price on every edit, which archives the product's default price (rejected by Stripe). It now swaps the price only when the amount changes.

Testing

• Unit: pnpm test passes 18/18 (11 new) covering the create, update, and delete actions; pnpm lint and tsc --noEmit are clean.
• Manual: verified creating a private lesson with no coach is rejected, reassigning the coach on edit saves successfully, and deleting an assigned coach shows an error.

Screenshots / Screencasts

Creating a private lesson without a coach is rejected:

Deleting a user assigned as coach to a private lesson is blocked:

Checklist

• Code is neat, readable, and works
• Code is commented where appropriate and w
• Commit messages follow our guidelines
• Issue number is linked
• Branch is linked
• Reviewers are assigned

Notes

The schema change isn't applied to the DB yet — run pnpm db:push. The CHECK constraint will fail to create if any existing private_lessons row has coach_id IS NULL, so backfill those rows with a coach (or soft-delete them) first.

Out of scope follow-up: editing a service's price still hits the Stripe "default price cannot be archived" error on a genuine price change; only the unchanged-price path is fixed here.

[RenaudBernier]

The error message just shows on first try to create service. It should show on every unsuccessful try.

Screen.Recording.2026-06-06.at.3.50.25.PM.mov

[maxiDeee]

Fixed bug for error message only showing up once, updated screenshot.
Please check the Notes section in PR description.

[RenaudBernier]

Looks good, works great!