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/tests/test_supplementary_batch.py

@@ -132,16 +132,23 @@ def test_one_query_per_table_and_latest_row_per_urn():
         "FactPupilCharacteristics": [],
         "FactDeprivation": [],
         "FactFinance": [],
+        "FactKs4Destinations": [],
+        "FactKs5Destinations": [],
     }
     session = _FakeSession(rows)
     out = get_supplementary_data_batch(session, [1, 2])
 
-    # Exactly one query per table — six total, regardless of two URNs.
+    # Exactly one query per table — eight total, regardless of two URNs.
     assert sorted(session.queries) == [
         "FactAdmissionDistance", "FactAdmissions", "FactDeprivation",
-        "FactFinance", "FactOfstedInspection", "FactPupilCharacteristics",
+        "FactFinance", "FactKs4Destinations", "FactKs5Destinations",
+        "FactOfstedInspection", "FactPupilCharacteristics",
     ]
 
+    # A school with no destination rows gets null, not an empty shell — the
+    # frontend renders the section from the block's presence.
+    assert out[1]["destinations"] is None
+
     # Latest Ofsted kept per URN
     assert out[1]["ofsted"]["overall_effectiveness"] == 2
     assert out[2]["ofsted"]["overall_effectiveness"] == 1