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>

CHANGELOG.md

@@ -5,6 +5,19 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Fixed
+
+- **A release image says which release it is.** The e2e gate in
+  `release.yml` rebuilt the image it was about to push with its own
+  `VERSION=e2e`, so every published image printed `e2e` for `-version` and in
+  the panel's footer, and the backup/restore version check compared a
+  backup's version with `e2e` - any backup restored into any release (audit
+  2026-10-12, H1). The gate now builds with the release's version
+  (`SELFPOST_E2E_VERSION`), the e2e checks `panel -version` and
+  `selfpost-backup -version` against it, and `release.yml` checks the stamp
+  again on the image right before pushing it. Images already published are
+  left as they are.
+
 ## [2.4.1] - 2026-10-12
 
 Documentation only; no product code changes.