ci: build + push the fleet-manager image (join the CI-built serving images)

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.

.gitea/workflows/gitea-main-ci.yaml

@@ -58,6 +58,7 @@ on:
       - ".gitea/workflows/ipxe-image.yaml"
       - ".gitea/workflows/dnsmasq-image.yaml"
       - ".gitea/workflows/spdk-image.yaml"
+      - ".gitea/workflows/fleet-manager-image.yaml"
 
 # One build per ref at a time; a newer push to the same branch cancels the
 # in-flight build (each build is a ~17-min VM run, so don't stack them).