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>

backend/models.py

@@ -189,6 +189,24 @@ class FactAdmissions(Base):
     admissions_policy = Column(String(100))
 
 
+class FactPupilCharacteristics(Base):
+    """School pupil composition from EES census — one row per URN per year."""
+    __tablename__ = "fact_pupil_characteristics"
+    __table_args__ = (
+        Index("ix_pupil_chars_urn_year", "urn", "year"),
+        MARTS,
+    )
+
+    urn = Column(Integer, primary_key=True)
+    year = Column(Integer, primary_key=True)
+    phase_type_grouping = Column(String(50))
+    total_pupils = Column(Integer)
+    female_pupils = Column(Integer)
+    male_pupils = Column(Integer)
+    fsm_pct = Column(Float)
+    eal_pct = Column(Float)
+
+
 class FactDeprivation(Base):
     """IDACI deprivation index — one row per URN."""
     __tablename__ = "fact_deprivation"