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/MetricTooltip.tsx

@@ -0,0 +1,31 @@
+'use client';
+
+import { METRIC_EXPLANATIONS } from '@/lib/metrics';
+import styles from './MetricTooltip.module.css';
+
+interface MetricTooltipProps {
+  metricKey?: string;
+  label?: string;
+  plain?: string;
+  detail?: string;
+}
+
+export function MetricTooltip({ metricKey, label, plain, detail }: MetricTooltipProps) {
+  const explanation = metricKey ? METRIC_EXPLANATIONS[metricKey] : undefined;
+  const tooltipLabel = label ?? explanation?.label;
+  const tooltipPlain = plain ?? explanation?.plain;
+  const tooltipDetail = detail ?? explanation?.detail;
+
+  if (!tooltipPlain) return null;
+
+  return (
+    <span className={styles.wrapper}>
+      <span className={styles.icon} aria-label={tooltipLabel ?? 'More information'} role="img">ⓘ</span>
+      <span className={styles.tooltip} role="tooltip">
+        {tooltipLabel && <span className={styles.tooltipLabel}>{tooltipLabel}</span>}
+        <span className={styles.tooltipPlain}>{tooltipPlain}</span>
+        {tooltipDetail && <span className={styles.tooltipDetail}>{tooltipDetail}</span>}
+      </span>
+    </span>
+  );
+}