feat(destinations): serve destinations without closing the gaps

The serialiser carries status through and computes no totals of its own.
The only aggregates in the payload are ones DfE published itself; whether
showing one is safe depends on how many of its components are suppressed,
which the frontend decides.

The batch guard grows from six tables to eight.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BvdDKvFFSZuMVDH5fEyTob

backend/models.py

@@ -321,3 +321,48 @@ class Ks2NationalAverage(Base):
     gps_high_pct = Column(Float)
     gps_avg_score = Column(Float)
     science_expected_pct = Column(Float)
+
+
+class FactKs4Destinations(Base):
+    """KS4 leavers destinations — one row per URN, year, pupil group, measure.
+
+    Long format rather than wide because pupil_group is a real third dimension.
+    `status` is load-bearing: 'suppressed' means DfE withheld a figure it
+    considered disclosive and the page must print "withheld"; 'not_applicable'
+    means the measure does not apply and the page must print nothing. `pupils`
+    is null for both, so collapsing status to a null check loses the
+    difference — and the categories sum to the cohort, so a consumer that
+    treats a withheld cell as zero republishes what DfE hid.
+    """
+    __tablename__ = "fact_ks4_destinations"
+    __table_args__ = (
+        Index("ix_ks4_dest_urn_year", "urn", "year"),
+        MARTS,
+    )
+
+    urn = Column(Integer, primary_key=True)
+    year = Column(Integer, primary_key=True)
+    pupil_group = Column(String(20), primary_key=True)
+    destination_measure = Column(String(40), primary_key=True)
+    cohort_pupils = Column(Integer)
+    pupils = Column(Integer)
+    percentage = Column(Float)
+    status = Column(String(20))
+
+
+class FactKs5Destinations(Base):
+    """16-18 study leavers destinations — same grain as FactKs4Destinations."""
+    __tablename__ = "fact_ks5_destinations"
+    __table_args__ = (
+        Index("ix_ks5_dest_urn_year", "urn", "year"),
+        MARTS,
+    )
+
+    urn = Column(Integer, primary_key=True)
+    year = Column(Integer, primary_key=True)
+    pupil_group = Column(String(20), primary_key=True)
+    destination_measure = Column(String(40), primary_key=True)
+    cohort_pupils = Column(Integer)
+    pupils = Column(Integer)
+    percentage = Column(Float)
+    status = Column(String(20))