forgejo-runner: gc every 30min instead of daily

- Fix template to use .Values.gc.schedule instead of hardcoded cron
- Change schedule from daily 03:00 UTC to every 30 minutes
- Prevents DinD PVC fill-up (was at 93% before manual prune)

k8s/infra/forgejo-runner/templates/gc-cronjob.yaml

@@ -43,7 +43,7 @@ metadata:
   labels:
     app: forgejo-runner-gc
 spec:
-  schedule: "0 2,5 * * *"      # Run at 02:00 and 05:00 UTC (more frequent for heavy build workloads)
+  schedule: {{ .Values.gc.schedule | quote }}
   concurrencyPolicy: Forbid
   successfulJobsHistoryLimit: 3
   failedJobsHistoryLimit: 3

k8s/infra/forgejo-runner/values.yaml

@@ -59,7 +59,7 @@ nodeSelector:
 # disable in per-runner overrides so it renders once.
 gc:
   enabled: true
-  schedule: "0 3 * * *"        # daily 03:00 UTC
+  schedule: "*/30 * * * *"     # every 30 minutes
   image: alpine/k8s:1.31.0
   pruneAge: "72h"              # Docker artifacts unused longer than this get pruned
   pruneAgeHours: 72             # Same as pruneAge but numeric for date arithmetic in shell