perf(places): index the reverse lookup, and isolate the registry in tests

Two review findings, both confirmed before fixing.

The client fixture in test_school_details.py patched load_school_data
but not _place_registry, which is a module-level cache. A probe settled
it rather than an argument: poisoning the global with a registry built
from data the fixture never saw, then issuing the fixture's own
request, returned that other dataset's places. So the new
`places == []` assertion was satisfied by a stale registry exactly as
well as by the fixture's own data, and proved nothing. Every other test
that touches place data already reset it; the fixture predates places
existing and was never updated. It resets it now.

places_for_urn walked every place in the registry and did a tuple
membership test against each, on /api/schools/{urn}, the site's
highest-traffic endpoint. It now reads a dict built once per registry.
Measured against a synthetic corpus of 27k schools in 1,650 places:
0.118ms per request becomes 0.0001ms, with the index built once in
21ms. Production carries ~5,000 places, so the scan there is larger
again. The absolute saving per request is small; the point is that it
is repeated on every school page view and costs nothing to remove.

The index is cached against the registry by identity rather than
behind a second flag. Anything that drops _place_registry — every test
that touches place data does — gets a fresh registry object, which no
longer matches what the index was built from, so the index rebuilds
with it. A separate _place_index = None would be one more thing to
forget, and a stale reverse index is precisely the first finding's bug
wearing a different hat.

That invalidation has its own test, and the test was checked by
breaking the identity check: five tests fail without it, so three
existing ones were already relying on it too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DXnXQKnPpZBBP61fBQiFkq

backend/tests/test_school_details.py

@@ -56,6 +56,11 @@ def client(monkeypatch):
     monkeypatch.setattr(
         app_module, "get_supplementary_data", lambda db, urn: {}
     )
+    # The place registry is a module-level cache, so without this the endpoint
+    # answers from whatever registry an earlier test happened to leave behind
+    # — and a `places == []` assertion is satisfied by a stale registry just
+    # as well as by this fixture's own data, which makes it prove nothing.
+    monkeypatch.setattr(app_module, "_place_registry", None)
     return TestClient(app_module.app, raise_server_exceptions=False)
 
 
@@ -212,3 +217,31 @@ def test_a_school_absent_from_the_phase_page_is_not_linked_to_it(monkeypatch):
     # The town publishes a primary page, but this secondary school is not on
     # it, and there are too few secondaries for a secondary page.
     assert town["phases"] == []
+
+
+def test_the_place_index_rebuilds_when_the_registry_is_replaced(monkeypatch):
+    """The reverse index is cached; a stale one would put another dataset's
+    places on a school page. Invalidation is an identity check against the
+    registry rather than a second flag, so this asserts the check works."""
+    from backend import app as app_module
+
+    monkeypatch.setattr(app_module, "_place_registry", None)
+    monkeypatch.setattr(app_module, "_place_index", None)
+    monkeypatch.setattr(app_module, "_place_index_source", None)
+    monkeypatch.setattr(app_module, "load_school_data", _brentwood_df("Primary"))
+
+    first = app_module.get_place_index()
+    assert 100000 in first
+
+    # Same registry object, so the index is reused rather than rebuilt.
+    assert app_module.get_place_index() is first
+
+    # Drop the registry the way every test that touches place data does. The
+    # index must follow it, not survive it.
+    app_module._place_registry = None
+    monkeypatch.setattr(app_module, "load_school_data",
+                        _brentwood_df("Primary", n=0))
+
+    rebuilt = app_module.get_place_index()
+    assert rebuilt is not first
+    assert 100000 not in rebuilt, "the index outlived the registry it came from"