fix(school): send the school page its admissions policy, and read it exactly

The header's Selective flag read school_info.admissions_policy, which the
detail endpoint never sent, so no school page could flag Selective while
its search row did (staging E2E: The Grammar School at Leeds). The detail
payload now carries it, and a contract test checks it carries every field
the header's flags read.

Sending it would have switched on two older copies of the tag logic #176
fixed in the rows. The Admissions section and the cut-off note both tested
includes('selective'), so every non-selective secondary would have read
"entry is by selective examination". The section also counted "None" as a
faith: Burntwood reads "a faith-based admissions priority (None)" today.
All of them now share isSelective() and hasReligiousCharacter(), which also
treats "Not applicable" as no faith, as the place table already does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app.py

@@ -1047,6 +1047,8 @@ async def get_school_details(request: Request, urn: int):
             "age_range": latest.get("age_range", ""),
             "has_sixth_form": latest.get("has_sixth_form"),
             "nursery_provision": latest.get("nursery_provision"),
+            # The header's Selective flag reads it (lib/schoolFacts).
+            "admissions_policy": latest.get("admissions_policy"),
             "status": latest.get("status"),
             "latitude": latest.get("latitude"),
             "longitude": latest.get("longitude"),