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.module.css

@@ -0,0 +1,83 @@
+.wrapper {
+  position: relative;
+  display: inline-flex;
+  align-items: center;
+  margin-left: 0.3em;
+}
+
+.icon {
+  font-size: 0.85em;
+  color: var(--text-muted, #8a7a72);
+  cursor: help;
+  line-height: 1;
+  user-select: none;
+  transition: color 0.15s ease;
+}
+
+.wrapper:hover .icon {
+  color: var(--accent-coral, #e07256);
+}
+
+.tooltip {
+  visibility: hidden;
+  opacity: 0;
+  position: absolute;
+  bottom: calc(100% + 6px);
+  left: 50%;
+  transform: translateX(-50%);
+  z-index: 9999;
+  width: 220px;
+  background: var(--bg-primary, #faf7f2);
+  border: 1px solid var(--border-color, #e8ddd4);
+  border-radius: 10px;
+  box-shadow: 0 4px 16px rgba(44, 36, 32, 0.15);
+  padding: 0.6rem 0.75rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.3rem;
+  pointer-events: none;
+  transition: opacity 0.15s ease, visibility 0.15s ease;
+}
+
+/* Keep tooltip visible when hovering over it */
+.wrapper:hover .tooltip {
+  visibility: visible;
+  opacity: 1;
+}
+
+/* Small arrow pointing down */
+.tooltip::after {
+  content: '';
+  position: absolute;
+  top: 100%;
+  left: 50%;
+  transform: translateX(-50%);
+  border: 5px solid transparent;
+  border-top-color: var(--border-color, #e8ddd4);
+}
+
+.tooltipLabel {
+  font-weight: 600;
+  font-size: 0.75rem;
+  color: var(--text-primary, #2c2420);
+}
+
+.tooltipPlain {
+  font-size: 0.75rem;
+  color: var(--text-secondary, #5a4a44);
+  line-height: 1.4;
+}
+
+.tooltipDetail {
+  font-size: 0.7rem;
+  color: var(--text-muted, #8a7a72);
+  line-height: 1.4;
+  margin-top: 0.1rem;
+}
+
+/* Flip tooltip below when near top of screen */
+@media (max-width: 480px) {
+  .tooltip {
+    width: 180px;
+  }
+}