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

backend/app.py

@@ -808,7 +808,13 @@ async def get_school_details(request: Request, urn: int):
         "census": supplementary.get("census"),
         "admissions": supplementary.get("admissions"),
         "admissions_history": supplementary.get("admissions_history") or [],
-        "admission_distance": supplementary.get("admission_distance"),
+        # Behind a flag, and withheld at the source rather than rendered-but-
+        # hidden: this endpoint is public and unauthenticated, so a field left
+        # in the payload is a published field. The key is absent, not null —
+        # null would state that this school has no cut-off, which is a
+        # different claim from "we are not publishing cut-offs".
+        **({"admission_distance": supplementary.get("admission_distance")}
+           if flags.is_enabled("admission_distance") else {}),
         "sen_detail": supplementary.get("sen_detail"),
         "phonics": supplementary.get("phonics"),
         "deprivation": supplementary.get("deprivation"),