ci: hard-fail PR and release scans on unacknowledged HIGH/CRITICAL CVEs (#484)

* ci: hard-fail PR and release scans on unacknowledged HIGH/CRITICAL CVEs

Make Trivy a real gate instead of an advisory signal:

- PR CI (docker-validate) no longer uses `continue-on-error: true` on the
  Trivy step, so any HIGH/CRITICAL finding not in `.trivyignore` fails the PR.
- The release pipeline (docker-publish.yml) now builds an amd64-only scan
  image into the local daemon before the multi-arch push-build, re-runs
  Trivy against that exact artifact, and only proceeds to the push if the
  scan passes. This closes the gap where a CVE landed between PR merge and
  release-time rebuild.
- New `.trivyignore` at repo root is the single source of truth for
  acknowledged CVEs across both workflows; it starts empty so the first CI
  run surfaces the full list, which we then populate with justifications.

* ci: populate .trivyignore with initial HIGH/CRITICAL acknowledgements

First CI run on the hard-fail Trivy policy surfaced 7 unacknowledged
findings. Each has been reviewed and justified inline:

- 6 CVEs in the statically-linked Go modules inside docker-compose v5.1.1
  (github.com/docker/docker, buildkit, otel/sdk x2, grpc). These are
  transitively bundled and cannot be bumped without an upstream Compose
  rebuild. The grpc CVE is already explicitly acknowledged in the
  Dockerfile rationale block at Dockerfile:108-111.
- 1 CVE in picomatch 4.0.3 bundled inside the npm CLI that ships with
  node:22-alpine. npm is only invoked at build time against our own
  package.json, so the ReDoS vector is not reachable.

Every entry has a revisit trigger (next Compose release or next Alpine
node bump).

.github/workflows/ci.yml

@@ -123,15 +123,15 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Scan image for vulnerabilities (Trivy)
+        # Hard-fails the PR on any HIGH or CRITICAL finding that is not listed
+        # in .trivyignore at the repo root. To accept a CVE, add it to that
+        # file with a human-readable justification comment, not here.
         uses: aquasecurity/trivy-action@57a97c7e7821a5776cebc9bb87c984fa69cba8f1 # v0.35.0
         with:
           image-ref: sencho:pr-test
           exit-code: '1'
           severity: 'CRITICAL,HIGH'
           format: 'table'
-        # Findings surface as a visible warning on the job without blocking the PR.
-        # Base-image CVEs are often outside our control - review manually before ignoring.
-        continue-on-error: true
 
   # ---------------------------------------------------------------------------
   # E2E Tests (PRs only, skipped for release-please PRs)