feat: add secondary school support with KS4 data and metric tooltips

- Backend: replace INNER JOIN ks2 with UNION ALL (ks2 + ks4) so primary
  and secondary schools both appear in the main DataFrame
- Backend: add /api/national-averages endpoint computing means from live
  data, replacing the hardcoded NATIONAL_AVG constant on the frontend
- Backend: add phase filter param to /api/schools; return phases from
  /api/filters; fix hardcoded "phase": "Primary" in school detail endpoint
- Backend: add KS4 metric definitions (Attainment 8, Progress 8, EBacc,
  English & Maths pass rates) to METRIC_DEFINITIONS and RANKING_COLUMNS
- Frontend: SchoolDetailView is now phase-aware — secondary schools show
  a GCSE Results section (Att8, P8, E&M, EBacc) instead of SATs; phonics
  tab hidden for secondary; admissions says Year 7 instead of Year 3;
  history table shows KS4 columns; chart datasets switch for secondary
- Frontend: new MetricTooltip component (CSS-only ⓘ icon) backed by
  METRIC_EXPLANATIONS — added to RWM, GPS, SEN, EAL, IDACI, progress
  scores and all KS4 metrics throughout SchoolDetailView and SchoolCard
- Frontend: METRIC_EXPLANATIONS extended with KS4 terms (Attainment 8,
  Progress 8, EBacc) and previously missing terms (SEN, EHCP, EAL, IDACI)
- Frontend: SchoolCard expands "RWM" to "Reading, Writing & Maths" and
  shows Attainment 8 / English & Maths Grade 4+ for secondary schools
- Frontend: FilterBar adds Phase dropdown (Primary / Secondary / All-through)
- Frontend: HomeView hero copy updated; compact list shows phase-aware metric
- Global metadata updated to remove "primary only" framing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/app/page.tsx

@@ -11,6 +11,7 @@ interface HomePageProps {
     search?: string;
     local_authority?: string;
     school_type?: string;
+    phase?: string;
     page?: string;
     postcode?: string;
     radius?: string;
@@ -19,7 +20,7 @@ interface HomePageProps {
 
 export const metadata = {
   title: 'Home',
-  description: 'Search and compare primary school KS2 performance across England',
+  description: 'Search and compare school performance across England',
 };
 
 // Force dynamic rendering (no static generation at build time)
@@ -38,6 +39,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
     params.search ||
     params.local_authority ||
     params.school_type ||
+    params.phase ||
     params.postcode
   );
 
@@ -52,6 +54,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
         search: params.search,
         local_authority: params.local_authority,
         school_type: params.school_type,
+        phase: params.phase,
         postcode: params.postcode,
         radius,
         page,
@@ -65,7 +68,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
     return (
       <HomeView
         initialSchools={schoolsData}
-        filters={filtersData || { local_authorities: [], school_types: [], years: [] }}
+        filters={filtersData || { local_authorities: [], school_types: [], years: [], phases: [] }}
         totalSchools={dataInfo?.total_schools ?? null}
       />
     );
@@ -76,7 +79,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
     return (
       <HomeView
         initialSchools={{ schools: [], page: 1, page_size: 50, total: 0, total_pages: 0 }}
-        filters={{ local_authorities: [], school_types: [], years: [] }}
+        filters={{ local_authorities: [], school_types: [], years: [], phases: [] }}
         totalSchools={null}
       />
     );