fix(api): filter by every GIAS phase, not just the grouped ones

/api/schools only recognised primary, secondary and all-through. Any
other phase the search page offers (nursery, 16 plus, middle deemed
primary/secondary) fell through to no filter, so "Nursery" returned the
whole result set, mostly primaries. Ungrouped phases now match exactly,
and an unknown phase returns nothing rather than everything.

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

e2e/tests/journeys.spec.ts

@@ -283,6 +283,20 @@ test('the phase filter switches straight from secondary to primary', async ({ pa
   await expect(phase).toHaveValue('primary');
 });
 
+test('a phase outside primary/secondary filters to that phase, not to everything', async ({ page }) => {
+  // The search page offers every GIAS phase, but the API only knew the grouped
+  // ones and silently dropped the rest — so "Nursery" returned primaries.
+  for (const phase of ['Nursery', '16 plus']) {
+    const res = await page.request.get(
+      `/api/schools?phase=${encodeURIComponent(phase.toLowerCase())}&page_size=50`);
+    expect(res.ok()).toBeTruthy();
+    const phases = new Set(
+      ((await res.json()).schools ?? []).map((s: { phase?: string }) => s.phase));
+    expect([...phases], `phase=${phase} returned other phases`)
+      .toEqual(phases.size ? [phase] : []);
+  }
+});
+
 test('a report-card school shows a Report Card badge in search results, not its old grade', async ({ page }) => {
   // List/map badges keyed off ofsted_grade (the carried-forward legacy grade)
   // and never reached the report-card branch, so report-card schools were