feat(places): name every authority a place sits in

SW19 is mostly Merton but partly Wandsworth, and the page said only Merton.
The cause was one field doing two jobs: _parent_authority takes the modal
authority, which is right for a 301 target and wrong as a statement about
where a place is.

This is not a corner case. A quarter of viable outcodes (425 of 1,760) and a
third of viable towns (263 of 783) cross an authority boundary — Bedford the
town spans Bedford and Central Bedfordshire.

Place now carries `authorities`, every authority holding at least a tenth of
the schools and at least two of them, largest first. parent_authority stays
single and unchanged, because a redirect still needs one target.

The share threshold exists because GIAS carries postcode errors: EN6 lists two
Shropshire schools among fourteen in Hertfordshire, and a bare "any authority
present" rule would print those as though they were real. A place too small or
too fragmented to clear the threshold still names its largest, so the page
never goes silent about where it is.

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

backend/tests/test_places.py

@@ -229,3 +229,64 @@ def test_no_curated_locality_names_a_london_borough():
         f"these are boroughs, not districts: {sorted(named)} - they already "
         "have an authority page covering every school"
     )
+
+
+def test_a_place_names_every_authority_it_straddles():
+    """SW19 is mostly Merton but partly Wandsworth.
+
+    A quarter of viable outcodes and a third of viable towns cross an
+    authority boundary, so naming only the largest asserts something false.
+    """
+    rows = (_town(26, "London", "Merton", start=300000)
+            + _town(7, "London", "Wandsworth", start=400000))
+    for r in rows:
+        r["postcode"] = "SW19 1AA"
+    reg = build_place_registry(_df(rows))
+
+    names = [n for n, _ in reg["outcode:sw19"].authorities]
+    assert names == ["Merton", "Wandsworth"]          # largest first
+    assert dict(reg["outcode:sw19"].authorities)["Wandsworth"] == 7
+
+
+def test_the_redirect_target_stays_a_single_authority():
+    # parent_authority and authorities do different jobs: a 301 needs one
+    # target, the page needs the truth.
+    rows = (_town(26, "London", "Merton", start=300000)
+            + _town(7, "London", "Wandsworth", start=400000))
+    for r in rows:
+        r["postcode"] = "SW19 1AA"
+    reg = build_place_registry(_df(rows))
+    assert reg["outcode:sw19"].parent_authority == "Merton"
+
+
+def test_a_stray_authority_below_the_share_threshold_is_not_named():
+    # GIAS carries postcode errors — EN6 lists two Shropshire schools among
+    # fourteen in Hertfordshire. Printing those as though real would be worse
+    # than omitting them.
+    rows = (_town(30, "Barnet", "Hertfordshire", start=300000)
+            + _town(1, "Barnet", "Shropshire", start=400000))
+    for r in rows:
+        r["postcode"] = "EN6 1AA"
+    reg = build_place_registry(_df(rows))
+    assert [n for n, _ in reg["outcode:en6"].authorities] == ["Hertfordshire"]
+
+
+def test_a_sentinel_authority_is_never_named():
+    rows = (_town(20, "London", "Merton", start=300000)
+            + _town(6, "London", "Does not apply", start=400000))
+    for r in rows:
+        r["postcode"] = "SW19 1AA"
+    reg = build_place_registry(_df(rows))
+    assert [n for n, _ in reg["outcode:sw19"].authorities] == ["Merton"]
+
+
+def test_a_place_always_names_at_least_one_authority():
+    # Even when every authority is below the share threshold, the page has to
+    # say where the place is.
+    rows = []
+    for i, la in enumerate(["A", "B", "C", "D", "E", "F", "G"]):
+        rows += _town(1, "Fragmented", la, start=300000 + i * 100)
+    reg = build_place_registry(_df(rows))
+    place = reg.get("town:fragmented")
+    assert place is not None
+    assert len(place.authorities) == 1