feat(renovate): add Gitea update automation

Run Renovate in Kubernetes to create reviewed image update PRs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/renovate-ci.yaml

@@ -0,0 +1,45 @@
+name: renovate-ci
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  validate-renovate:
+    runs-on: [self-hosted, linux, arch, homelab]
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Validate Renovate Compose draft
+        shell: bash
+        run: |
+          set -euo pipefail
+          trap 'rm -f renovate/.env' EXIT
+          printf '%s\n' \
+            'RENOVATE_ENDPOINT=https://gitea.example/api/v1' \
+            'RENOVATE_TOKEN=test-token' \
+            'RENOVATE_REPOSITORIES=forust/homelab' \
+            > renovate/.env
+          docker compose -f renovate/renovate-compose.yaml config --quiet
+
+      - name: Validate Kubernetes manifests
+        shell: bash
+        run: |
+          set -euo pipefail
+          for manifest in renovate/k8s/namespace.yaml renovate/k8s/configmap.yaml renovate/k8s/cronjob.yaml; do
+            kubectl apply --dry-run=client --validate=false -f "$manifest" >/dev/null
+          done
+
+      - name: Validate Renovate repository config
+        shell: bash
+        run: |
+          set -euo pipefail
+          docker run --rm \
+            -v "$PWD:/work" \
+            -w /work \
+            renovate/renovate:44.83.2 \
+            renovate-config-validator renovate.json