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/routers/tasks.py

@@ -18,7 +18,7 @@ from api.schemas.task import (
     TaskStatusBulkSyncRead,
     TaskUpdate,
 )
-from api.services.legacy_compat import meter_legacy_query_param
+from api.services.legacy_compat import meter_legacy_body_from_model, meter_legacy_query_param
 from api.services.lifecycle import status_value, transition_task_status
 from api.task_status import normalize_task_status
 
@@ -105,9 +105,20 @@ async def count_tasks(
 
 @router.post("/", response_model=TaskRead, status_code=status.HTTP_201_CREATED)
 async def create_task(
+    request: Request,
+    response: Response,
     body: TaskCreate,
     session: AsyncSession = Depends(get_session),
 ) -> Task:
+    await meter_legacy_body_from_model(
+        body,
+        session=session,
+        request=request,
+        response=response,
+        method="POST",
+        route="/tasks/",
+        replacement_ref="POST /tasks/ with workplan_id",
+    )
     task = Task(**body.model_dump())
     session.add(task)
     if status_value(task.status) == "progress":