Vikunja 2.6.0 deprecates `service.jwtsecret` in favor of `service.secret`;
switch the env var name (secret payload unchanged) and fix the admin-promotion
example in the README to the current CLI (`user set-admin --admin`).
Co-Authored-By: Paperclip <noreply@paperclip.ing>
Namespace-scoped Postgres 16 + unified Vikunja container behind Traefik
ingress at vikunja.basicstack.de. Both pods pinned to Hetzner fsn1 due to
their encrypted PVCs (10 Gi DB, 10 Gi attachments). Local auth disabled;
sign-in only via Pocket ID (client `Vikunja`, groups `vikunja_admins` /
`vikunja_members`). SMTP mailer left disabled pending mailbox provisioning.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
