fix(places): a locality collision must not break the sitemap

Sitemap regeneration failed on staging. 'richmond' in the curated locality
list collides with the GIAS town Richmond in North Yorkshire (37 schools), the
registry raised, and the admin endpoint 500d — taking down sitemap generation
for all 25,000 school pages over one bad row of curated data.

The guard now skips the colliding locality and logs an error. Skipping still
achieves what the guard was for — a locality never silently shadows a town —
without letting curated data break the site. That matters beyond this bug:
GIAS town names change with no code change here, so a raise could fire
spontaneously in production later.

Also removes four localities that were London boroughs rather than districts.
Hackney, Islington, Greenwich and Ealing are local authorities with 104, 72,
108 and 115 schools and already have authority pages; a locality defined by
two or three outcodes would have been a partial near-duplicate of one — the
thin-content failure the two-namespace design exists to avoid. A test now
guards the whole borough list.

Validated against the live corpus: 15 localities, no town collisions, no
authority duplicates, all 15 clear the threshold.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/tests/test_places.py

@@ -108,16 +108,42 @@ def test_locality_below_the_threshold_is_not_published(monkeypatch):
     assert "locality:nowhere" not in reg
 
 
-def test_a_locality_may_not_shadow_a_viable_town(monkeypatch):
-    # Silently shadowing a town would lose a page carrying real demand.
+def test_a_locality_may_not_shadow_a_viable_town(monkeypatch, caplog):
+    """A colliding locality is skipped loudly, and the town survives.
+
+    This used to raise, which took down sitemap generation for all 25,000
+    school pages the first time a curated slug met a real GIAS town. Curated
+    data must not be able to break the site — and GIAS town names change with
+    no code change at all, so the raise could fire spontaneously.
+    """
+    import logging
+
     from backend import localities
     monkeypatch.setattr(localities, "LOCALITY_OUTCODES",
                         {"brentwood": ("Brentwood", ("CM13",))})
     rows = _town(MIN_SCHOOLS, "Brentwood", "Essex")
     for r in rows:
         r["postcode"] = "CM13 1AA"
-    with pytest.raises(ValueError, match="brentwood"):
-        build_place_registry(_df(rows))
+
+    with caplog.at_level(logging.ERROR):
+        reg = build_place_registry(_df(rows))
+
+    assert "locality:brentwood" not in reg      # skipped
+    assert "town:brentwood" in reg              # the town is untouched
+    assert "brentwood" in caplog.text           # and it was loud about it
+
+
+def test_a_locality_collision_does_not_break_the_rest_of_the_registry(monkeypatch):
+    # The whole point of skipping rather than raising.
+    from backend import localities
+    monkeypatch.setattr(localities, "LOCALITY_OUTCODES",
+                        {"brentwood": ("Brentwood", ("CM13",))})
+    rows = _town(MIN_SCHOOLS, "Brentwood", "Essex")
+    for r in rows:
+        r["postcode"] = "CM13 1AA"
+    reg = build_place_registry(_df(rows))
+    assert "authority:essex" in reg
+    assert "outcode:cm13" in reg
 
 
 def test_outcode_places_are_built_from_postcodes():
@@ -174,3 +200,32 @@ def test_the_pipeline_seed_mirrors_the_canonical_module():
         for row in csv.DictReader(seed_path.open())
     }
     assert seed == LOCALITY_OUTCODES
+
+
+def test_no_curated_locality_names_a_london_borough():
+    """Boroughs are authorities and already have a page.
+
+    A locality defined by two or three outcodes inside a borough would be a
+    partial, near-duplicate subset of that authority page — the exact
+    thin-content failure the two-namespace design exists to avoid. Hackney,
+    Islington, Greenwich and Ealing were all in the first draft.
+
+    Hardcoded rather than read from the corpus because this must fail in CI,
+    where there is no database.
+    """
+    from backend.localities import LOCALITY_OUTCODES
+
+    boroughs = {
+        "barking-and-dagenham", "barnet", "bexley", "brent", "bromley",
+        "camden", "croydon", "ealing", "enfield", "greenwich", "hackney",
+        "hammersmith-and-fulham", "haringey", "harrow", "havering",
+        "hillingdon", "hounslow", "islington", "kensington-and-chelsea",
+        "kingston-upon-thames", "lambeth", "lewisham", "merton", "newham",
+        "redbridge", "richmond-upon-thames", "southwark", "sutton",
+        "tower-hamlets", "waltham-forest", "wandsworth", "westminster",
+    }
+    named = boroughs & set(LOCALITY_OUTCODES)
+    assert not named, (
+        f"these are boroughs, not districts: {sorted(named)} - they already "
+        "have an authority page covering every school"
+    )