Add a weekly Renovate workflow for dependency updates

Runs renovate/renovate against this Gitea instance on the native
deploy runner, opening PRs for outdated Bundler dependencies instead
of relying on manually noticing (as happened with puma/mustermann
earlier).

.gitea/workflows/renovate.yml

@@ -0,0 +1,24 @@
+name: Renovate
+
+on:
+  workflow_dispatch:
+  schedule:
+    # Gitea extends standard cron syntax with an optional "TZ=" prefix.
+    - cron: "TZ=Europe/Bratislava 0 6 * * 1"
+
+jobs:
+  renovate:
+    # The native deploy runner (see docs/deployment.adoc) already has
+    # Docker access — Renovate itself is a one-shot batch job, not a
+    # long-running service, so `docker run`, not docker-compose.
+    runs-on: deploy
+    steps:
+      - name: Run Renovate
+        run: |
+          docker run --rm \
+            -e RENOVATE_PLATFORM=gitea \
+            -e RENOVATE_ENDPOINT="${{ gitea.server_url }}/api/v1/" \
+            -e RENOVATE_TOKEN="${{ secrets.RENOVATE_TOKEN }}" \
+            -e RENOVATE_REPOSITORIES="${{ gitea.repository }}" \
+            -e RENOVATE_ONBOARDING=false \
+            renovate/renovate