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/decisions.py

@@ -15,7 +15,7 @@ from api.events import EventEnvelope, publish_event
 from api.models.decision import Decision, DecisionStatus, DecisionType
 from api.models.progress_event import ProgressEvent
 from api.schemas.decision import DecisionCreate, DecisionRead, DecisionResolve, DecisionUpdate
-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
 
 router = APIRouter(prefix="/decisions", tags=["decisions"])
 
@@ -75,9 +75,20 @@ async def list_decisions(
 
 @router.post("/", response_model=DecisionRead, status_code=status.HTTP_201_CREATED)
 async def create_decision(
+    request: Request,
+    response: Response,
     body: DecisionCreate,
     session: AsyncSession = Depends(get_session),
 ) -> Decision:
+    await meter_legacy_body_from_model(
+        body,
+        session=session,
+        request=request,
+        response=response,
+        method="POST",
+        route="/decisions/",
+        replacement_ref="POST /decisions/ with workplan_id",
+    )
     data = body.model_dump()
     note = _needs_escalation(body)
     if note: