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/tasks.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import date
 
-from fastapi import APIRouter, Depends, HTTPException, Query, status
+from fastapi import APIRouter, Depends, HTTPException, Query, Request, Response, status
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -18,6 +18,7 @@ from api.schemas.task import (
     TaskStatusBulkSyncRead,
     TaskUpdate,
 )
+from api.services.legacy_compat import meter_legacy_query_param
 from api.services.lifecycle import status_value, transition_task_status
 from api.task_status import normalize_task_status
 
@@ -26,6 +27,8 @@ router = APIRouter(prefix="/tasks", tags=["tasks"])
 
 @router.get("/", response_model=list[TaskRead])
 async def list_tasks(
+    request: Request,
+    response: Response,
     workplan_id: uuid.UUID | None = None,
     workstream_id: uuid.UUID | None = None,
     status: str | None = None,
@@ -37,6 +40,15 @@ async def list_tasks(
     offset: int = Query(0, ge=0),
     session: AsyncSession = Depends(get_session),
 ) -> list[Task]:
+    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="/tasks/",
+            replacement_ref="/tasks/?workplan_id=<workplan_id>",
+        )
     q = select(Task)
     scope_id = workplan_id or workstream_id
     if scope_id:
@@ -62,11 +74,22 @@ async def list_tasks(
 
 @router.get("/counts", response_model=list[TaskCountRead])
 async def count_tasks(
+    request: Request,
+    response: Response,
     workplan_id: uuid.UUID | None = None,
     workstream_id: uuid.UUID | None = None,
     status: str | None = None,
     session: AsyncSession = Depends(get_session),
 ) -> list[TaskCountRead]:
+    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="/tasks/counts",
+            replacement_ref="/tasks/counts?workplan_id=<workplan_id>",
+        )
     q = select(Task.workplan_id, Task.status, func.count()).group_by(Task.workplan_id, Task.status)
     scope_id = workplan_id or workstream_id
     if scope_id: