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/decisions.py

@@ -4,7 +4,7 @@ import uuid
 from datetime import datetime, timezone
 from pathlib import Path
 
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, Request, Response, status
 
 logger = logging.getLogger(__name__)
 from sqlalchemy import select
@@ -15,6 +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
 
 router = APIRouter(prefix="/decisions", tags=["decisions"])
 
@@ -39,6 +40,8 @@ def _needs_escalation(body: DecisionCreate) -> str | None:
 
 @router.get("/", response_model=list[DecisionRead])
 async def list_decisions(
+    request: Request,
+    response: Response,
     topic_id: uuid.UUID | None = None,
     workplan_id: uuid.UUID | None = None,
     workstream_id: uuid.UUID | None = None,
@@ -46,6 +49,15 @@ async def list_decisions(
     decision_type: DecisionType | None = None,
     session: AsyncSession = Depends(get_session),
 ) -> list[Decision]:
+    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="/decisions/",
+            replacement_ref="/decisions/?workplan_id=<workplan_id>",
+        )
     q = select(Decision)
     if topic_id:
         q = q.where(Decision.topic_id == topic_id)