release: stamp the image with the release's version (audit H1)

The e2e gate rebuilt selfpost:e2e through compose.override.yml with its
own VERSION=e2e, and that rebuild is the image release.yml pushed: every
published image printed "e2e", and the backup/restore version check
compared a backup's version with "e2e".

- compose.override.yml: VERSION: ${SELFPOST_E2E_VERSION:-e2e}.
- release.yml: the e2e step gets SELFPOST_E2E_VERSION = the release's
  X.Y.Z; a new step before the push checks panel -version and
  selfpost-backup -version of selfpost:e2e against it.
- e2e: image_stamped_with_version checks both binaries against
  expectedVersion() (the variable, or "e2e" when unset).
- development.md, CHANGELOG [Unreleased], audit plan line ticked,
  roadmap 1/61.

Evidence: make e2e on the test server for this tree with
SELFPOST_E2E_VERSION=2.4.2-stamp - exit 0, TestE2E/image_stamped_with_version
PASS, and `docker run --rm --entrypoint <bin> selfpost:e2e -version` prints
2.4.2-stamp for panel and selfpost-backup. GOOS=linux go vet in test/e2e
clean. Published images are left as they are.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/stack.go

@@ -89,6 +89,16 @@ func (s *stack) execIn(service string, args ...string) (string, error) {
 	return string(out), err
 }
 
+// expectedVersion is the version the stand's image is built with: the
+// release's X.Y.Z when release.yml runs the suite, "e2e" for a local run. Its
+// default must stay the default of VERSION in compose.override.yml.
+func expectedVersion() string {
+	if v := os.Getenv("SELFPOST_E2E_VERSION"); v != "" {
+		return v
+	}
+	return "e2e"
+}
+
 func (s *stack) build() error {
 	_, err := s.compose("build")
 	return err