feat(api): serve similar schools on the detail endpoint

Rides in the existing payload rather than a new endpoint: the page already
makes one server fetch for its data, and /school/[slug] regenerates weekly,
so the per-request cost is paid once per school per week.

Wrapped so a failure in selection never 500s a page that is otherwise
complete — the posture get_supplementary_data already takes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_similar_schools.py

@@ -250,3 +250,49 @@ def test_values_are_json_safe_native_types():
         assert isinstance(school["urn"], int)
         assert isinstance(school["distance_miles"], float)
         assert not isinstance(school["metric_value"], np.generic)
+
+
+# ---------------------------------------------------------------------------
+# The endpoint
+# ---------------------------------------------------------------------------
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+def _endpoint_frame():
+    return _frame(
+        _row(100001, "Subject Primary"),
+        _row(100002, "Neighbour A", latitude=_at(0.5)),
+        _row(100003, "Neighbour B", latitude=_at(0.6)),
+    )
+
+
+@pytest.fixture()
+def client(monkeypatch):
+    from backend import app as app_module
+
+    monkeypatch.setattr(app_module, "load_latest_school_data", _endpoint_frame)
+    monkeypatch.setattr(app_module, "load_school_data", _endpoint_frame)
+    monkeypatch.setattr(app_module, "get_supplementary_data", lambda db, urn: {})
+    return TestClient(app_module.app, raise_server_exceptions=False)
+
+
+def test_detail_payload_carries_similar_schools(client):
+    resp = client.get("/api/schools/100001")
+    assert resp.status_code == 200, resp.text
+    similar = resp.json()["similar_schools"]
+    assert [s["school_name"] for s in similar] == ["Neighbour A", "Neighbour B"]
+    assert similar[0]["metric_key"] == "rwm_expected_pct"
+
+
+def test_a_failure_in_selection_does_not_break_the_page(client, monkeypatch):
+    from backend import app as app_module
+
+    def _explode(*args, **kwargs):
+        raise ValueError("selection blew up")
+
+    monkeypatch.setattr(app_module, "select_similar", _explode)
+    resp = client.get("/api/schools/100001")
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["similar_schools"] == []