STATE-WP-0069 T04: meter workstream_id query-param aliases

Add legacy_compat helper and Deprecation headers when callers filter
/tasks, /decisions, /token-events, or /execution/launch-requests with
workstream_id. Preferred workplan_id filters are unchanged. Route removal
remains gated on legacy-meter zero-usage windows.

api/routers/execution.py

@@ -28,6 +28,7 @@ from api.services.execution_queue import (
     workplan_blockers,
 )
 from api.routers.workstreams import _legacy_key, _meter_legacy_route
+from api.services.legacy_compat import meter_legacy_query_param
 from api.workplan_status import CLOSED_WORKPLAN_STATUSES, normalize_workplan_status
 
 router = APIRouter(prefix="/execution", tags=["execution"])
@@ -193,13 +194,26 @@ async def create_launch_request(
 
 @router.get("/launch-requests", response_model=list[LaunchRequestRead])
 async def list_launch_requests(
+    request: Request,
+    response: Response,
+    workplan_id: uuid.UUID | None = None,
     workstream_id: uuid.UUID | None = None,
     request_status: str | None = None,
     session: AsyncSession = Depends(get_session),
 ) -> list[WorkplanLaunchRequest]:
+    if workstream_id is not None and workplan_id is None:
+        await meter_legacy_query_param(
+            session=session,
+            request=request,
+            response=response,
+            method="GET",
+            route="/execution/launch-requests",
+            replacement_ref="/execution/launch-requests?workplan_id=<workplan_id>",
+        )
     q = select(WorkplanLaunchRequest).order_by(WorkplanLaunchRequest.created_at.desc())
-    if workstream_id:
-        q = q.where(WorkplanLaunchRequest.workplan_id == workstream_id)
+    scope_id = workplan_id or workstream_id
+    if scope_id:
+        q = q.where(WorkplanLaunchRequest.workplan_id == scope_id)
     if request_status:
         q = q.where(WorkplanLaunchRequest.status == request_status)
     result = await session.execute(q)