fix: preserve encryption key across helm upgrades using lookup

Use lookup to check for an existing Secret before generating a new
random key. Priority order:
  1. values.yaml n8n.encryptionKey (explicit)
  2. Existing Secret in the cluster (upgrade-safe)
  3. randAlphaNum 32 (first install only)

This prevents the "Mismatching encryption keys" error caused by
randAlphaNum generating a new key on every helm upgrade.
Same stable-value pattern applied to basicAuth password.

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: "2.19.2-a"
+version: "2.19.2-b"
 appVersion: "2.19.2"
 keywords:
   - n8n