feat(places): London localities and postcode districts

The GIAS town field puts 1,819 London schools under the single value
'London', so it cannot answer 'schools in Battersea' — a query that appears in
the baseline. No single field can: parliamentary constituency gives Battersea
but not Canary Wharf, admin_ward gives Canary Wharf but not Battersea, and
neither gives Clapham or Shoreditch. So a locality is curated, defined by the
postcode districts it covers, which needs no new ingestion.

A locality may not shadow a published town: the registry raises rather than
silently costing a page that carries real demand. One below the threshold is
logged rather than raising, because a locality can legitimately be too small.

The pipeline seed mirrors the module, with a test guarding the drift — the
same arrangement gias_codes has, and for the same reason: the backend image
does not contain pipeline/.

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

backend/tests/test_places.py

@@ -84,3 +84,93 @@ def test_a_school_is_counted_once_even_with_several_years_of_rows():
         rows += [{**r, "year": year} for r in _town(MIN_SCHOOLS, "Beccles", "Suffolk")]
     reg = build_place_registry(_df(rows))
     assert len(reg["town:beccles"].urns) == MIN_SCHOOLS
+
+
+def test_locality_groups_schools_by_outcode(monkeypatch):
+    # The GIAS town field collapses 1,819 London schools into "London", so a
+    # locality is defined by its postcode districts instead.
+    from backend import localities
+    monkeypatch.setattr(localities, "LOCALITY_OUTCODES",
+                        {"battersea": ("Battersea", ("SW11",))})
+    rows = _town(MIN_SCHOOLS, "London", "Wandsworth")
+    for r in rows:
+        r["postcode"] = "SW11 2AA"
+    reg = build_place_registry(_df(rows))
+    assert reg["locality:battersea"].name == "Battersea"
+    assert len(reg["locality:battersea"].urns) == MIN_SCHOOLS
+
+
+def test_locality_below_the_threshold_is_not_published(monkeypatch):
+    from backend import localities
+    monkeypatch.setattr(localities, "LOCALITY_OUTCODES",
+                        {"nowhere": ("Nowhere", ("ZZ99",))})
+    reg = build_place_registry(_df(_town(MIN_SCHOOLS, "London", "Wandsworth")))
+    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.
+    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))
+
+
+def test_outcode_places_are_built_from_postcodes():
+    rows = _town(MIN_SCHOOLS, "Brentwood", "Essex")
+    for r in rows:
+        r["postcode"] = "CM13 1AA"
+    reg = build_place_registry(_df(rows))
+    assert reg["outcode:cm13"].name == "CM13"
+    assert len(reg["outcode:cm13"].urns) == MIN_SCHOOLS
+
+
+def test_malformed_postcodes_do_not_create_places():
+    rows = _town(MIN_SCHOOLS, "Brentwood", "Essex")
+    for r in rows:
+        r["postcode"] = "not a postcode"
+    reg = build_place_registry(_df(rows))
+    assert not any(k.startswith("outcode:") for k in reg)
+
+
+def test_every_curated_locality_is_structurally_valid():
+    # Guards the hand-maintained file: real slug, real name, real outcodes.
+    import re
+    from backend.localities import LOCALITY_OUTCODES
+
+    assert LOCALITY_OUTCODES, "the curated locality list must not be empty"
+    for slug, (name, outcodes) in LOCALITY_OUTCODES.items():
+        assert re.fullmatch(r"[a-z0-9-]+", slug), slug
+        assert name.strip() == name and name, slug
+        assert outcodes, f"{slug} has no outcodes"
+        for oc in outcodes:
+            assert re.fullmatch(r"[A-Z]{1,2}\d{1,2}[A-Z]?", oc), (slug, oc)
+
+
+def test_the_pipeline_seed_mirrors_the_canonical_module():
+    """Two copies with no drift guard is worse than one copy.
+
+    backend/localities.py is canonical because the backend image does not
+    contain pipeline/. The seed exists so the warehouse can join on the same
+    definitions, and this is what stops the two diverging — the same
+    arrangement assert_gias_code_names_match_seed.sql gives gias_codes.
+    """
+    import csv
+    from pathlib import Path
+
+    from backend.localities import LOCALITY_OUTCODES
+
+    seed_path = (Path(__file__).resolve().parents[2]
+                 / "pipeline/transform/seeds/locality_outcodes.csv")
+    assert seed_path.exists(), f"missing seed mirror at {seed_path}"
+
+    seed = {
+        row["locality_slug"]: (row["locality_name"],
+                               tuple(row["outcodes"].split("|")))
+        for row in csv.DictReader(seed_path.open())
+    }
+    assert seed == LOCALITY_OUTCODES