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

e2e/tests/journeys.spec.ts

@@ -1885,3 +1885,29 @@ test('no page title repeats the brand', async ({ page }) => {
     expect(brands, `${path} repeats the brand: ${title}`).toBeLessThanOrEqual(1);
   }
 });
+
+test('a place straddling a boundary names every authority it sits in', async ({ page }) => {
+  // A quarter of outcodes and a third of towns cross an authority boundary —
+  // SW19 is mostly Merton but partly Wandsworth. Naming only the largest
+  // asserts something false about the place.
+  const { places } = await (await page.request.get('/api/places')).json();
+  const outcode = places.find((p: { kind: string }) => p.kind === 'outcode');
+  expect(outcode).toBeTruthy();
+
+  // Find any place the registry reports as straddling.
+  let straddling: { kind: string; slug: string } | null = null;
+  for (const p of places.filter((p: { kind: string }) => p.kind === 'outcode').slice(0, 40)) {
+    const d = await (await page.request.get(`/api/places/outcode/${p.slug}`)).json();
+    if ((d.place.authorities ?? []).length > 1) { straddling = p; break; }
+  }
+  test.skip(!straddling, 'no straddling outcode found in the sample');
+
+  const detail = await (await page.request.get(
+    `/api/places/outcode/${straddling!.slug}`)).json();
+  await page.goto(`/schools/near/${straddling!.slug}`);
+
+  for (const a of detail.place.authorities) {
+    await expect(page.locator(`a[href="/schools/authority/${a.slug}"]`).first())
+      .toBeVisible();
+  }
+});