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>

scripts/ci/release.py

@@ -0,0 +1,136 @@
+"""Release identity checks and promotion of the exact digests that passed E2E.
+
+Uses only the standard library and Docker Buildx. No registry mutation happens
+until every image in the set has been resolved and its build labels validated.
+"""
+import argparse
+import json
+import os
+from pathlib import Path
+import re
+import subprocess
+import time
+from urllib.request import Request, urlopen
+
+COMPONENTS = ('BACKEND', 'FRONTEND', 'PIPELINE')
+
+
+def docker(*args):
+    return subprocess.check_output(['docker', 'buildx', 'imagetools', *args], text=True).strip()
+
+
+def check_sha(sha):
+    if not re.fullmatch(r'[0-9a-f]{40}', sha):
+        raise ValueError('Expected a full commit SHA')
+    return sha
+
+
+def check_digest(digest):
+    if not re.fullmatch(r'sha256:[0-9a-f]{64}', digest):
+        raise ValueError('Expected an immutable image digest')
+    return digest
+
+
+def image_identity(ref):
+    image = json.loads(docker('inspect', ref, '--format', '{{json .Image}}'))
+    configs = [image] if 'config' in image else list(image.values())
+    identities = set()
+    for config in configs:
+        labels = config['config']['Labels']
+        identities.add((labels['io.schoolcompare.commit'], labels['io.schoolcompare.build-id']))
+    if len(identities) != 1:
+        raise ValueError('Image platforms disagree about their release identity')
+    return next(iter(identities))
+
+
+def resolve_images(sha, verified=False, expected_build_id=None):
+    check_sha(sha)
+    refs = []
+    build_ids = set()
+    for component in COMPONENTS:
+        image = f"{os.environ['REGISTRY']}/{os.environ[component + '_IMAGE_NAME']}"
+        if verified:
+            manifest = json.loads(docker('inspect', f'{image}:verified-{sha}', '--format', '{{json .Manifest}}'))
+            digest = check_digest(manifest['digest'])
+        else:
+            digest = check_digest(os.environ[component + '_DIGEST'])
+        ref = f'{image}@{digest}'
+        actual_sha, build_id = image_identity(ref)
+        if actual_sha != sha or not re.fullmatch(r'[0-9a-f]{32}', build_id):
+            raise ValueError(f'Unrecognised release identity for {component}')
+        if expected_build_id is not None and build_id != expected_build_id:
+            raise ValueError(f'Build identity mismatch for {component}')
+        build_ids.add(build_id)
+        refs.append((image, ref))
+    if len(build_ids) != 1:
+        raise ValueError('Refusing a mixed image set')
+    return {'sha': sha, 'build_id': build_ids.pop(), 'images': refs}
+
+
+def verify(sha, build_id):
+    release = resolve_images(sha, expected_build_id=build_id)
+    for image, ref in release['images']:
+        docker('create', '-t', f'{image}:verified-{sha}', ref)
+    return release
+
+
+def promote(sha):
+    release = resolve_images(sha, verified=True)
+    # Resolve all targets first; never discover a missing candidate halfway through.
+    for image, _ in release['images']:
+        try:
+            docker('create', '-t', f'{image}:prod-previous', f'{image}:prod')
+        except subprocess.CalledProcessError:
+            print(f'No rollback pointer saved for {image}', flush=True)
+    for image, ref in release['images']:
+        docker('create', '-t', f'{image}:prod', ref)
+    return release
+
+
+def matches(payload, sha, build_id):
+    return all(payload.get(component) == {'sha': sha, 'build_id': build_id}
+               for component in ('frontend', 'backend'))
+
+
+def wait(base_url, sha, build_id, timeout):
+    check_sha(sha)
+    if not re.fullmatch(r'[0-9a-f]{32}', build_id):
+        raise ValueError('Missing expected build identity')
+    deadline = time.monotonic() + timeout
+    while time.monotonic() < deadline:
+        try:
+            req = Request(f'{base_url.rstrip("/")}/release.json?check={time.time_ns()}',
+                          headers={'Cache-Control': 'no-cache'})
+            with urlopen(req, timeout=min(10, max(.1, deadline - time.monotonic()))) as response:
+                payload = json.load(response)
+            if matches(payload, sha, build_id):
+                print(f'Verified deployed release {sha} / {build_id}')
+                return
+        except (OSError, ValueError):
+            pass
+        time.sleep(min(5, max(0, deadline - time.monotonic())))
+    raise RuntimeError('Deployment did not report the expected frontend/backend release')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('action', choices=['wait', 'verify', 'promote'])
+    parser.add_argument('--timeout', type=float, default=300)
+    parser.add_argument('--release', type=Path)
+    parser.add_argument('--output', type=Path)
+    args = parser.parse_args()
+    identity = json.loads(args.release.read_text()) if args.release else {
+        'sha': os.environ.get('EXPECTED_SHA', ''),
+        'build_id': os.environ.get('EXPECTED_BUILD_ID', ''),
+    }
+    if args.action == 'wait':
+        wait(os.environ['BASE_URL'], identity['sha'], identity['build_id'], args.timeout)
+        return
+    result = (verify(identity['sha'], identity['build_id']) if args.action == 'verify'
+              else promote(identity['sha']))
+    if args.output:
+        args.output.write_text(json.dumps(result))
+
+
+if __name__ == '__main__':
+    main()