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>

templates/deployment.yaml

@@ -41,6 +41,8 @@ spec:
           containerPort: 5678
           protocol: TCP
         env:
+        - name: NODE_OPTIONS
+          value: "--max-old-space-size={{ .Values.nodeHeapSizeMb }}"
         - name: N8N_PORT
           value: "5678"
         - name: N8N_PROTOCOL
@@ -48,7 +50,7 @@ spec:
         - name: N8N_HOST
           value: {{ .Values.n8n.host | quote }}
         {{- if .Values.n8n.webhookUrl }}
-        - name: WEBHOOK_URL
+        - name: N8N_WEBHOOK_URL
           value: {{ .Values.n8n.webhookUrl | quote }}
         {{- end }}
         - name: GENERIC_TIMEZONE