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/app.py

@@ -919,6 +919,7 @@ async def get_school_details(request: Request, urn: int):
         "phonics": supplementary.get("phonics"),
         "deprivation": supplementary.get("deprivation"),
         "finance": supplementary.get("finance"),
+        "destinations": supplementary.get("destinations"),
     }