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/SchoolCard.tsx

@@ -48,12 +48,35 @@ export function SchoolCard({ school, onAddToCompare, onRemoveFromCompare, showDi
         )}
       </div>
 
-      {(school.rwm_expected_pct !== null || school.reading_progress !== null) && (
+      {(school.rwm_expected_pct != null || school.attainment_8_score != null || school.reading_progress !== null) && (
         <div className={styles.metrics}>
+          {/* KS4 card metrics for secondary schools */}
+          {school.attainment_8_score != null && (
+            <div className={styles.metric}>
+              <span className={styles.metricLabel}>
+                Attainment 8
+                <span className={styles.metricHint}>avg grade across best 8 GCSEs</span>
+              </span>
+              <div className={styles.metricValue}>
+                <strong>{school.attainment_8_score.toFixed(1)}</strong>
+              </div>
+            </div>
+          )}
+          {school.english_maths_standard_pass_pct != null && (
+            <div className={styles.metric}>
+              <span className={styles.metricLabel}>
+                English &amp; Maths Grade 4+
+                <span className={styles.metricHint}>% standard pass in both</span>
+              </span>
+              <div className={styles.metricValue}>
+                <strong>{formatPercentage(school.english_maths_standard_pass_pct)}</strong>
+              </div>
+            </div>
+          )}
           {school.rwm_expected_pct !== null && (
             <div className={styles.metric}>
               <span className={styles.metricLabel}>
-                RWM Expected
+                Reading, Writing &amp; Maths
                 <span className={styles.metricHint}>% meeting expected standard</span>
               </span>
               <div className={styles.metricValue}>