fix(charts/linkwarden): Use user-defined existing secret for database

[adrian-salas]

What this PR does / why we need it

This PR updates the Helm chart’s deployment template to respect a user-provided existingSecret for the database URI, falling back to the auto-generated secret name only if none is specified

Special notes for your reviewer

• The change is backward-compatible: if no existingSecret is provided, the chart behaves as before

[adrian-salas]

Might actually be fixed by: #40

For the context, when setting values like so

linkwarden: 
  database: 
    existingSecret: "linkwarden-postgresql-app"

the deploment is set like so

spec:
  containers:
  - env:
    - name: DATABASE_URL
      valueFrom:
        secretKeyRef:
          key: uri
          name: linkwarden-db

Even when set, the value is using the helper.tpl value

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions]

false