docker/metadata-action's type=semver strips the leading 'v', so releases
published only :3.45.0 / :3.83.1-beta.0. But git tags + GitHub releases
are named v3.45.0, so anyone pinning ghcr.io/.../backend:v3.45.0 (the
obvious choice) hit 'manifest unknown' — exactly #664.
Add v-prefixed semver patterns (v{{version}}, v{{major}}.{{minor}},
v{{major}}) alongside the existing bare ones, for both backend and
frontend. Now both :v3.45.0 and :3.45.0 resolve.
Applies to future releases; the already-published v3.45.0 only has the
bare :3.45.0 tag (retagging past releases is out of scope).