Business app stream controls

The platform endpoints for Business class stream controls are ready. The paired
mobile UI is still in [draft Business app PR #69](https://github.com/Mister-Shamir-ApS/bookingbible.app/pull/69),
so staff will not see the controls in a released Business build until that PR is
reviewed, merged, and included in a mobile rollout. The draft pins the endpoint
and DTO contract in `src/api/hooks/admin-checkin-endpoints.ts`, consumes it from
`src/api/hooks/use-admin-checkin-ops.ts`, and presents the workflow from
`src/components/checkin/stream-control-sheet.tsx` and
`app/(business)/(tabs)/checkin.tsx` (draft head `31378df`).

The draft's schedule notification controls have also been verified against the
platform's exact delivery contract. Cancellation clients can use Email, SMS, or
Push; cancellation instructors can use Email or SMS; substitution clients can
use Push; and substitute instructors can use Email. The combined audience and
channel selection in `src/components/schedule/substitute-sheet.tsx` is mapped by
the platform to those exact recipient-specific legs, while unsupported
combinations are rejected before mutation. PR #69 remains release-gated by its
native dependency cooldown, signed builds, and physical-device testing rather
than by a schedule contract mismatch.

The paired design shows only safe source names and readiness, never stream keys,
ingest URLs, provider ids, playback links, or source credentials. Go-live and
source changes repeat the platform streaming module, entitlement, venue setting,
streamability, date, permission, and instructor go-live gates before changing
anything.

These controls are operationally silent: they do not send client notifications.
Ending a stream completes the class first, then stops the provider source only
when the shared-stream guard says it is safe, and reports whether the provider
source was stopped, left on for another class, or needs attention.

If provider shutdown needs attention after completion, an authorized staff
member can retry only that provider step. The retry never completes the class
again or repeats its class-side effects, stays tenant-bound, and never exposes
the other class occurrence that kept a shared stream running.

Cross-app evidence: Consumer and Branded remain watch-only for this change. Their
existing playback contracts stay in `src/api/hooks/livestream-endpoints.ts`,
`app/(consumer)/player.tsx`, and `app/(branded)/player.tsx`; PR #69 adds no
Consumer or Branded operational stream controls. Release owners should verify
the draft PR status and the Business build train before announcing app
availability.