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

nextjs-app/__tests__/components/PlaceView.test.tsx

@@ -168,3 +168,47 @@ describe('PlaceView presentation', () => {
       .not.toBeInTheDocument();
   });
 });
+
+describe('PlaceView authorities', () => {
+  const straddling: PlaceDetail = {
+    place: { kind: 'outcode', slug: 'sw19', name: 'SW19', count: 33,
+             parent_authority: 'Merton', phases: ['primary'],
+             authorities: [
+               { name: 'Merton', slug: 'merton', count: 26 },
+               { name: 'Wandsworth', slug: 'wandsworth', count: 7 },
+             ] },
+    schools: [
+      { urn: 1, school_name: 'Alpha Primary', phase: 'Primary',
+        rwm_expected_pct: 82, attainment_8_score: null } as never,
+    ],
+    averages: { rwm_expected_pct: 63, attainment_8_score: null },
+  };
+
+  it('names every authority the place straddles, not just the largest', () => {
+    // SW19 is mostly Merton but partly Wandsworth. Naming one asserts
+    // something false about a quarter of outcodes.
+    render(<PlaceView detail={straddling} englandAverage={61} neighbours={[]} />);
+    expect(screen.getByRole('link', { name: 'Merton' }))
+      .toHaveAttribute('href', '/schools/authority/merton');
+    expect(screen.getByRole('link', { name: 'Wandsworth' }))
+      .toHaveAttribute('href', '/schools/authority/wandsworth');
+  });
+
+  it('joins them readably rather than as a bare list', () => {
+    // Asserted on the summary line's whole text: a loose /and/ matcher also
+    // hits "Wandsworth".
+    const { container } = render(<PlaceView detail={straddling}
+      englandAverage={61} neighbours={[]} />);
+    const summary = container.querySelector('header p');
+    expect(summary?.textContent).toContain('Merton and Wandsworth');
+  });
+
+  it('falls back to the single parent when the field is absent', () => {
+    // A cached API response predating the authorities field must not blank
+    // the line entirely.
+    const legacy = { ...straddling,
+      place: { ...straddling.place, authorities: undefined } };
+    render(<PlaceView detail={legacy} englandAverage={61} neighbours={[]} />);
+    expect(screen.getByRole('link', { name: 'Merton' })).toBeInTheDocument();
+  });
+});