feat(ci): gate promotion on the image set that actually passed E2E

Staging health polling asked only whether something answered HTTP 200 at
the base URL. It could not tell the new deployment from the old one, so
journeys could pass against the previous release, and concurrent merges
could move the staging tags underneath a run in flight.

Each staging run now mints a build ID and stamps all three images with
the commit and that ID, as labels and — for frontend and backend — as a
build-time JSON file that environment overrides cannot rewrite.
/release.json reports both identities uncached, and scripts/ci/release.py
polls for the expected pair before and after the journeys. Only then are
the captured build digests tagged verified-<sha>.

Promotion resolves those verified tags to immutable digests, revalidates
their labels, and refuses a mixed or incomplete set before any :prod tag
moves. The whole staging workflow shares one concurrency group with
cancellation disabled, so releases serialise.

The scripts are stdlib-only and unit-tested against mocked registry and
HTTP behaviour; PR checks now run the pipeline and CI suites too. The
runbook records what this cannot prove locally, and that the first
rollout needs a commit built by this workflow.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nextjs-app/Dockerfile

@@ -28,6 +28,10 @@ ENV NODE_ENV=production
 ARG FASTAPI_URL=http://backend:80/api
 ENV FASTAPI_URL=${FASTAPI_URL}
 
+ARG BUILD_SHA=development
+ARG BUILD_ID=development
+RUN node -e 'require("fs").writeFileSync("build-info.json", JSON.stringify({sha:process.argv[1],build_id:process.argv[2]}))' "$BUILD_SHA" "$BUILD_ID"
+
 # Build application
 RUN npm run build
 
@@ -70,6 +74,12 @@ USER nextjs
 EXPOSE 3000
 
 # Set environment variables
+ARG BUILD_SHA=development
+ARG BUILD_ID=development
+LABEL io.schoolcompare.build-id=$BUILD_ID
+LABEL io.schoolcompare.commit=$BUILD_SHA
+COPY --from=builder /app/build-info.json ./build-info.json
+
 ENV PORT=3000
 ENV HOSTNAME="0.0.0.0"