fix(06): WR-03 fix locust load test document list shape mismatch

The GET /api/documents/ response is {items: [...], total: N, ...} not a
bare list. Change docs = resp.json() to resp.json().get("items", []) so
get_document task correctly accesses docs[0]["id"] without TypeError.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/load_tests/locustfile.py

@@ -77,7 +77,7 @@ class DocuVaultUser(HttpUser):
             name="GET /api/documents/ (for get_document)",
         )
         if resp.status_code == 200:
-            docs = resp.json()
+            docs = resp.json().get("items", [])
             if docs:
                 doc_id = docs[0]["id"]
                 self.client.get(