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/registry-gc.yaml

@@ -64,7 +64,7 @@ jobs:
       DISPATCH_DRY: ${{ inputs.dry_run }}
       GC_ENABLE: ${{ vars.REGISTRY_GC_ENABLE }}
       # Only images built by THIS repo's workflows.
-      IMAGES: "fleet-boot fleet-ipxe fleet-dnsmasq spdk-nvmf"
+      IMAGES: "fleet-boot fleet-ipxe fleet-dnsmasq spdk-nvmf fleet-manager"
     steps:
       - name: Reconcile registry against live branches
         # The runner defaults `run:` to sh (dash); this script uses bashisms