Add .gitea/workflows/fleet-manager-image.yaml, modelled on dnsmasq-image.yaml:
buildah bud of serving/fleet-manager (multi-stage SPA+Go→distroless) on a
path filter, with the same branch/tag tagging scheme (semver on tags, main-<sha>
+ latest on main, <branch>-<sha> dev images). Wires it into the shared
conventions:
- gitea-main-ci.yaml paths-ignore: list the new workflow so a fleet-manager
image change doesn't trigger the ~17-min UKI build (serving/** already covers
the dir; the .gitea/ workflow file needed listing).
- registry-gc.yaml IMAGES: add fleet-manager so its dev-branch builds are GC'd.
