ci: harden supply chain (SHA pin actions, least-privilege permissions) (#475)

Pin three third-party actions to full commit SHAs with version comments
so dependabot can update them in place without exposing the pipeline to
upstream tag tampering:

  - aquasecurity/trivy-action@v0.35.0
  - peter-evans/create-pull-request@v8.1.1
  - googleapis/release-please-action@v4.4.0

Add a workflow-level permissions: contents: read default to ci.yml, with
update-screenshots keeping its explicit contents: write + pull-requests:
write override. Add the same default to docker-publish.yml (single job).

Expand .dockerignore to explicitly exclude .env*, *.key, *.pem, .github/,
docs/, website/, e2e/, tests/, test-results/, and playwright-report/ as
defense in depth against accidental build-context leaks. None of these
paths are read by any COPY step in Dockerfile.

.github/workflows/docker-publish.yml

@@ -15,6 +15,8 @@ jobs:
     name: Push Docker image to Docker Hub
     runs-on: ubuntu-latest
     timeout-minutes: 30
+    permissions:
+      contents: read
     steps:
       - name: Check out the repo
         uses: actions/checkout@v6