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/app.py

@@ -38,7 +38,7 @@ from .data_loader import (
 )
 from .data_loader import get_data_info as get_db_info
 from . import flags
-from .places import build_place_registry, places_for_urn
+from .places import build_place_index, build_place_registry, places_for_urn
 from .schemas import METRIC_DEFINITIONS, RANKING_COLUMNS, SCHOOL_COLUMNS
 from .utils import clean_for_json, convert_to_native
 
@@ -65,6 +65,10 @@ _sitemaps: dict[str, str] | None = None
 # Built from the same DataFrame the sitemap uses, so places and sitemap can
 # never describe different corpora. Reset by the same admin endpoint.
 _place_registry: dict | None = None
+# Cached beside the registry, and invalidated by identity against it — see
+# get_place_index. Never cleared independently.
+_place_index: dict | None = None
+_place_index_source: dict | None = None
 
 VALID_PLACE_KINDS = ("town", "locality", "authority", "outcode")
 
@@ -188,6 +192,24 @@ def get_place_registry() -> dict:
     return _place_registry
 
 
+def get_place_index() -> dict:
+    """URN → its published places, cached against the registry it came from.
+
+    Invalidation is an identity check rather than a second flag to remember to
+    clear. Anything that drops `_place_registry` — the tests all do — gets a
+    fresh registry object here, which no longer matches the one 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 exactly the
+    bug that would put links to another dataset's places on a school page.
+    """
+    global _place_index, _place_index_source
+    registry = get_place_registry()
+    if _place_index is None or _place_index_source is not registry:
+        _place_index = build_place_index(registry)
+        _place_index_source = registry
+    return _place_index
+
+
 def _urlset(rows: list[str]) -> str:
     return "\n".join([
         '<?xml version="1.0" encoding="UTF-8"?>',
@@ -229,7 +251,7 @@ def _places_payload(urn: int) -> list[dict]:
     so they report no phase links on their own.
     """
     payload = []
-    for place in places_for_urn(get_place_registry(), int(urn)):
+    for place in places_for_urn(get_place_index(), int(urn)):
         phases = [
             {
                 "phase": phase,