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>

docs/audit-2026-10-12.md

@@ -545,7 +545,7 @@ has already run is never edited — the next file is added; the published
 
 ### Stage 0 — release hygiene (no product code) → `2.4.2`
 
-- [ ] **Stamp the right version** (H1). Make the e2e build take the version from
+- [x] **Stamp the right version** (H1). Make the e2e build take the version from
    the environment (`VERSION: ${SELFPOST_E2E_VERSION:-e2e}` in
    `compose.override.yml`, set by `release.yml`), or have the e2e reuse the
    image already built instead of rebuilding. Done when the e2e asserts