fix: increase memory limit to 1Gi and set NODE_OPTIONS heap size

n8n 2.23+ has higher memory requirements. The previous 512Mi limit
caused OOM crashes (JS heap exhausted at ~248MB).

Changes:
- memory limit: 512Mi -> 1Gi, request: 256Mi -> 512Mi
- NODE_OPTIONS: --max-old-space-size={{ nodeHeapSizeMb }} (default 768MB)
  keeps JS heap at 75% of limit, leaving headroom for native code/buffers
- WEBHOOK_URL -> N8N_WEBHOOK_URL (deprecation warning fix)
- values.yaml: expose nodeHeapSizeMb for tuning

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

Chart.yaml

@@ -2,7 +2,7 @@ apiVersion: v2
 name: n8n
 description: A Helm chart for n8n workflow automation on Kubernetes (ARM/Raspberry Pi ready)
 type: application
-version: "2.36.5"
+version: "2.36.5-a"
 appVersion: "2.36.5"
 keywords:
   - n8n