feat: Add an optional scheduled backup CronJob to the chart

Off by default: a backup with nowhere durable to land is a false sense of
safety and the chart cannot know where that is. NOTES.txt says so when it
is off.

No restore manifest ships: a restore must never be something a helm
upgrade can trigger.

.gitea/workflows/chart-release.yml

@@ -76,6 +76,13 @@ jobs:
                   fi
                   echo "ok: reaper configured in cloud mode only"
 
+            - name: Render with backups enabled
+              run: |
+                  helm template test "$CHART_DIR" \
+                    --set backup.enabled=true \
+                    --set backup.image=gitea.hostxtra.co.uk/mrhid6/vantage/vantagectl:latest \
+                    --set backup.pvcName=vantage-backups > /dev/null
+
             - name: Render against external Redis and MongoDB
               run: |
                   helm template test "$CHART_DIR" \
@@ -149,6 +156,12 @@ jobs:
                       --set ingress.enabled=true \
                       --set ingress.web.host=vantage.example.com \
                       --set ingress.grpc.host=agents.example.com
+                  refuses "backup enabled with no pvcName" \
+                      --set backup.enabled=true \
+                      --set backup.image=gitea.hostxtra.co.uk/mrhid6/vantage/vantagectl:latest
+                  refuses "backup enabled with no image" \
+                      --set backup.enabled=true \
+                      --set backup.pvcName=vantage-backups
 
             - name: Read the chart version
               id: chart