Publish version tags (e.g. v0.33.5) on the self-host Docker images

Right now, self-hosters can only pin images to sha-<commit> or follow latest. Following latest means database migrations and compose changes land without review. Pinning a SHA is safe, but update tools like Renovate, Diun, and What's Up Docker can't put SHA tags in order. They can't tell that a newer release exists, so every upgrade means checking GitHub releases, mapping the release to its commit, and confirming that the tag exists on Docker Hub. Version tags would also help with #792: those tools can already alert on new semver tags.

Proposed solution

Have image-to-docker-hub.yml add a version tag next to latest and sha-*, reading the version from packages/frontend/package.json (which release-version-check.yml already checks against the release PR title):

• frontend: add v${VERSION} to the image-tag list passed to frontend-docker-build

• backend: add -t "$IMAGE:v${VERSION}" to the imagetools create call in merge-backend

Every successful build on main produces images, so a hotfix pushed to main without a version bump would move that version's tag to the new commit. That seems acceptable, since it would still be the latest build of that version, and sha-* stays available for exact pinning.