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/components/HomeView.tsx

@@ -51,8 +51,8 @@ export function HomeView({ initialSchools, filters, totalSchools }: HomeViewProp
       {/* Combined Hero + Search and Filters */}
       {!isSearchActive && (
         <div className={styles.heroSection}>
-          <h1 className={styles.heroTitle}>Compare Primary School Performance</h1>
-          <p className={styles.heroDescription}>Search and compare KS2 results for thousands of schools across England</p>
+          <h1 className={styles.heroTitle}>Compare School Performance</h1>
+          <p className={styles.heroDescription}>Search and compare SATs and GCSE results for thousands of schools across England</p>
         </div>
       )}
 
@@ -64,7 +64,7 @@ export function HomeView({ initialSchools, filters, totalSchools }: HomeViewProp
       {/* Discovery section shown on landing page before any search */}
       {!isSearchActive && initialSchools.schools.length === 0 && (
         <div className={styles.discoverySection}>
-          {totalSchools && <p className={styles.discoveryCount}><strong>{totalSchools.toLocaleString()}+</strong> primary schools across England</p>}
+          {totalSchools && <p className={styles.discoveryCount}><strong>{totalSchools.toLocaleString()}+</strong> primary and secondary schools across England</p>}
           <p className={styles.discoveryHints}>Try searching for a school name, or enter a postcode to find schools near you.</p>
           <div className={styles.quickSearches}>
             <span className={styles.quickSearchLabel}>Quick searches:</span>
@@ -258,7 +258,14 @@ function CompactSchoolItem({ school, onAddToCompare, isInCompare }: CompactSchoo
         </div>
         <div className={styles.compactItemStats}>
           <span className={styles.compactStat}>
-            <strong>{school.rwm_expected_pct !== null ? `${school.rwm_expected_pct}%` : '-'}</strong> RWM
+            <strong>
+              {school.attainment_8_score != null
+                ? school.attainment_8_score.toFixed(1)
+                : school.rwm_expected_pct !== null
+                  ? `${school.rwm_expected_pct}%`
+                  : '-'}
+            </strong>{' '}
+            {school.attainment_8_score != null ? 'Att 8' : 'RWM'}
           </span>
           <span className={styles.compactStat}>
             <strong>{school.total_pupils || '-'}</strong> pupils