feat(api): retire phase-1/2 /workstreams REST aliases with 410 stranglers

Add retire_legacy_route() helper and convert CRUD, dependency, and
execution intent legacy routes to 410 Gone while preserving final-call
legacy-meter attribution. Update tests, retirement backlog, and close
STATE-WP-0075 with handoff to STATE-WP-0070 T02 and STATE-WP-0073 T04.

api/services/legacy_compat.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 
 import logging
 
-from fastapi import Request, Response
+from fastapi import HTTPException, Request, Response, status
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from api.services.legacy_meter import identity_from_request, record_legacy_usage
@@ -97,6 +97,37 @@ async def meter_legacy_body_field(
         logger.warning("legacy-meter failed to record %s", interface_key, exc_info=True)
 
 
+async def retire_legacy_route(
+    *,
+    session: AsyncSession,
+    request: Request | None,
+    response: Response | None,
+    interface_key: str,
+    replacement_ref: str,
+    detail: str,
+) -> None:
+    """Record final legacy-meter usage for this route, then return 410 Gone."""
+    mark_legacy_response(response, replacement_ref)
+    try:
+        await record_legacy_usage(
+            session,
+            interface_key=interface_key,
+            interface_kind="rest_api",
+            replacement_ref=replacement_ref,
+            owner_component=_LEGACY_OWNER,
+            replacement_verified=True,
+            identity=identity_from_request(request),
+        )
+    except Exception:
+        await session.rollback()
+        logger.warning("legacy-meter failed to record %s", interface_key, exc_info=True)
+    raise HTTPException(
+        status_code=status.HTTP_410_GONE,
+        detail=detail,
+        headers=legacy_response_headers(replacement_ref),
+    )
+
+
 async def meter_legacy_body_from_model(
     body: object,
     *,