ci: pin black and flake8, let the bandit hook fail, cache npm in the JS job (#352)

The lint job installs black==26.10.1 and flake8==7.4.1 from
requirements-dev.txt (the versions CI resolved on main today), so a new
release cannot turn CI red with no code change.

The local bandit hook ran 'bash -c ... "$@"; true': it could never
fail, and bash bound the first staged file to $0 so it was never
scanned. It now runs 'bandit -lll -q' directly, failing on high
severity exactly like code-quality.yml.

The frontend modules job's setup-node caches npm keyed on
tools/vendor/package-lock.json.

.github/workflows/test.yml

@@ -221,7 +221,10 @@ jobs:
       - uses: actions/setup-python@v7
         with:
           python-version: "3.12"
-      - run: pip install black flake8
+          cache: pip
+          cache-dependency-path: requirements-dev.txt
+      # The exact black and flake8 pins from requirements-dev.txt, so a new release cannot turn CI red.
+      - run: pip install $(grep -E '^(black|flake8)==' requirements-dev.txt)
       - run: black --check .
       - run: flake8 .
 
@@ -235,6 +238,8 @@ jobs:
       - uses: actions/setup-node@v7
         with:
           node-version: "24"
+          cache: npm
+          cache-dependency-path: tools/vendor/package-lock.json
 
       - name: Check shipped extension and page modules parse
         run: |