chore: add 5-day dependency cooling period for supply-chain hardening (#7718)

.github/workflows/test-backend.yml

@@ -13,6 +13,10 @@ jobs:
 
     env:
       PRODUCTION: false
+      # Install from the committed lockfile; never re-resolve. The rolling
+      # `exclude-newer` cooling window (pyproject [tool.uv]) would otherwise make
+      # every uv command re-resolve and fail on in-window pins.
+      UV_FROZEN: "1"
 
     strategy:
       fail-fast: true