feat(detail): show pupil gender split on school detail pages

Upgrades the existing "Pupils" stat to include a compact split bar and
percentage hint for mixed schools (single-sex schools already carry a
"Boys's/Girls's school" badge, so the split would be redundant).

Wires fact_pupil_characteristics into the API: new SQLAlchemy model and
a real census block in /api/schools/{urn} replacing the prior null stub.

On the primary detail page the inline "Pupils: 241" text is replaced by
a richer block (display number + bar + "52% girls · 48% boys"). On the
secondary detail page the existing "Total pupils" hero stat card grows
the bar and hint beneath the number. Both fall back to the previous
text-only rendering when census gender data is missing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/SecondarySchoolDetailView.module.css

@@ -884,6 +884,45 @@
     margin-top: 0;
 }
 
+/* Gender split — attached beneath the Total pupils stat value */
+.genderBar {
+    display: flex;
+    height: 4px;
+    border-radius: 999px;
+    overflow: hidden;
+    background: rgba(0, 0, 0, 0.08);
+    margin-top: 0.45rem;
+}
+
+.genderBarGirls {
+    background: #b45778;
+}
+
+.genderBarBoys {
+    background: var(--accent-teal, #2d7d7d);
+}
+
+.genderSplitHint {
+    font-size: 0.7rem;
+    color: var(--text-muted, #6d685f);
+    margin-top: 0.35rem;
+    font-weight: 500;
+}
+
+.genderSplitGirls {
+    color: #b45778;
+    font-weight: 600;
+}
+
+.genderSplitBoys {
+    color: var(--accent-teal, #2d7d7d);
+    font-weight: 600;
+}
+
+.genderSplitSep {
+    color: var(--border-color, #e5dfd5);
+}
+
 /* ── Attainment 8 visual bar ─────────────────────────── */
 .att8Viz {
     margin: 1.25rem 0 0.5rem;