ci: scope server workflow to server paths; harden deploy push

Switch docker-build.yml from paths-ignore to an explicit paths allowlist
so the workflow only fires on changes to solitaire_server/, solitaire_sync/,
solitaire_core/, Cargo.toml, Cargo.lock, or the workflow file itself.

Also harden the "commit and push updated kustomization" step:
- exit 0 early when the kustomization has no staged diff (nothing to push)
- retry the pull+push loop up to 3 times with a 5 s delay to handle
  concurrent pushes that race the CI commit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -70,6 +70,9 @@ jobs:
           git config user.email "ci@gitea.local"
           git config user.name "Gitea CI"
           git add deploy/kustomization.yaml
-          git commit -m "chore(deploy): bump image to ${{ steps.meta.outputs.sha }} [skip ci]" || true
-          git pull --rebase origin master
-          git push
+          git diff --cached --quiet && exit 0  # nothing to commit — skip push
+          git commit -m "chore(deploy): bump image to ${{ steps.meta.outputs.sha }} [skip ci]"
+          for i in 1 2 3; do
+            git pull --rebase origin master && git push && break
+            sleep 5
+          done