STATE-WP-0069 T04/T06: body metering and workplan-first state internals

Share LegacyWorkstreamIdBodyMixin across create schemas; meter POST /tasks/
and /decisions/ workstream_id bodies. State summary uses workplan flow;
NextStep dual-writes workplan_* fields alongside legacy workstream_*.

api/services/legacy_compat.py

@@ -84,4 +84,28 @@ async def meter_legacy_body_field(
         )
     except Exception:
         await session.rollback()
-        logger.warning("legacy-meter failed to record %s", interface_key, exc_info=True)
+        logger.warning("legacy-meter failed to record %s", interface_key, exc_info=True)
+
+
+async def meter_legacy_body_from_model(
+    body: object,
+    *,
+    session: AsyncSession,
+    request: Request | None,
+    response: Response | None,
+    method: str,
+    route: str,
+    replacement_ref: str,
+    field: str = "workstream_id",
+) -> None:
+    if not getattr(body, "used_legacy_workstream_id", False):
+        return
+    await meter_legacy_body_field(
+        session=session,
+        request=request,
+        response=response,
+        method=method,
+        route=route,
+        replacement_ref=replacement_ref,
+        field=field,
+    )