fix(search): start postcode searches at half a mile

New postcode searches, the near-me button and URLs without a radius now
use 0.5 miles. A postcode URL with no radius used to show "1 mile" in the
Distance control while the API applied its own 5-mile default; the page
and the map fetch now send the same default the control displays.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

e2e/tests/journeys.spec.ts

@@ -270,6 +270,9 @@ test('searching by name returns school results', async ({ page }) => {
 test('searching by postcode returns nearby schools', async ({ page }) => {
   await searchByName(page, 'B1 1BB');
   await expect(schoolLinks(page).first()).toBeVisible({ timeout: 15_000 });
+  // A new postcode search starts at half a mile.
+  await expect(page).toHaveURL(/[?&]radius=0\.5(&|$)/);
+  await expect(page.getByRole('combobox', { name: 'Distance' })).toHaveValue('0.5');
 });
 
 test('the phase filter switches straight from secondary to primary', async ({ page }) => {