feat(flags): ship last-distance-offered dark behind a flag

One gate, at the source. The frontend needs no change: DistanceSection
already returns null when distance_m is missing, and the admissions
block already conditions on (admissions || admissionDistance). Only 57
local authorities publish cut-offs, so the off-path is the commonest
path on the site and is well covered already.

Absent, not null. /api/schools/ is public and unauthenticated, so a
field left in the payload is a published field — the reasoning already
recorded in c9a1892 when history was withheld. The two are also
different claims: null says this school has no cut-off, absent says
cut-offs are not being published at all. The frontend type now says so.

The feature is on main and live on staging and has never reached
production, which is what makes it the right first consumer: the flag
lets the code promote without the feature appearing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/lib/types.ts

@@ -361,7 +361,12 @@ export interface SchoolDetailsResponse {
    * held back as a paid feature and are not part of this public payload — see
    * data_loader._admission_distance.
    */
-  admission_distance: SchoolAdmissionDistance | null;
+  /**
+   * Absent — not null — when the admission_distance flag is off. Null means
+   * "this school has no published cut-off"; absent means "cut-offs are not
+   * being published at all". They are different claims and the type says so.
+   */
+  admission_distance?: SchoolAdmissionDistance | null;
   deprivation: SchoolDeprivation | null;
   finance: SchoolFinance | null;
 }