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/compose.override.yml

@@ -28,7 +28,11 @@ services:
       context: ../../..
       dockerfile: build/Dockerfile
       args:
-        VERSION: e2e
+        # release.yml sets SELFPOST_E2E_VERSION to the release's X.Y.Z, so the
+        # image this build tags selfpost:e2e - the one release.yml then pushes -
+        # carries the release's version, not "e2e". A local `make e2e` leaves it
+        # unset. The suite checks the stamp (image_stamped_with_version).
+        VERSION: ${SELFPOST_E2E_VERSION:-e2e}
     image: selfpost:e2e
     environment:
       SELFPOST_HOSTNAME: mail.e2e.test