fix(places): submit and link the phase variants

/schools/[place]/[phase] shipped as routes but reached nothing. The sitemap
emitted one URL per registry entry and the registry had no phase dimension, so
~950 pages were absent from every sitemap — and PlaceView did not link them
either, leaving them reachable by nothing at all.

That is the query shape the baseline actually showed: 'primary schools in
beccles', 'secondary schools in brentwood'. Publishing the routes without a
path in meant building for the demand and then hiding from it.

Place now carries phase_urns so the per-phase threshold can be applied without
re-querying, the sitemap emits a variant wherever a phase clears the threshold
on its own, and the API exposes the qualifying phases so the place page links
only variants that exist. Outcodes are excluded: nobody searches 'primary
schools in SW11' and those routes do not exist.

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

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

@@ -4,7 +4,7 @@ import type { PlaceDetail } from '@/lib/places';
 
 const detail: PlaceDetail = {
   place: { kind: 'town', slug: 'brentwood', name: 'Brentwood', count: 29,
-           parent_authority: 'Essex' },
+           parent_authority: 'Essex', phases: ['primary'] },
   schools: [
     { urn: 1, school_name: 'Alpha Primary', rwm_expected_pct: 82,
       ofsted_grade: 1 } as never,
@@ -91,3 +91,24 @@ describe('PlaceView structured data', () => {
     }
   });
 });
+
+describe('PlaceView phase variants', () => {
+  it('links the phase variants that exist', () => {
+    render(<PlaceView detail={detail} englandAverage={61} neighbours={[]} />);
+    expect(screen.getByRole('link', { name: /Primary schools in Brentwood/i }))
+      .toHaveAttribute('href', '/schools/brentwood/primary');
+  });
+
+  it('links no variant for a phase below its own threshold', () => {
+    render(<PlaceView detail={detail} englandAverage={61} neighbours={[]} />);
+    expect(screen.queryByRole('link', { name: /Secondary schools in Brentwood/i }))
+      .not.toBeInTheDocument();
+  });
+
+  it('does not link sideways from a variant page to itself', () => {
+    render(<PlaceView detail={detail} phase="primary" englandAverage={61}
+      neighbours={[]} />);
+    expect(screen.queryByRole('link', { name: /Primary schools in Brentwood/i }))
+      .not.toBeInTheDocument();
+  });
+});