feat(chart): support existingSecret for DB credentials (Doppler / External Secrets / Vault compatible)

[danielqb]

Problem

MariaDB and GLPI database credentials were only configurable as plaintext
values in values.yaml (mariadb.auth.rootPassword/username/password, etc),
always rendered into chart-managed Secrets. There was no way to point the
chart at a Secret already populated by an external secrets manager
(Doppler, External Secrets Operator, Vault, Sealed Secrets, etc), which
is the standard pattern the Bitnami charts support via
'.auth.existingSecret'.

Fix

Added two independent existingSecret overrides, following the Bitnami
naming convention:

• mariadb.auth.existingSecret: for the MariaDB server's own credentials
  (needs MARIADB_ROOT_PASSWORD, MARIADB_DATABASE, MARIADB_USER,
  MARIADB_PASSWORD). Used by the MariaDB StatefulSet and the
  mariadb-timezone Job. When set, mariadb-secret.yaml is skipped.

• glpi.database.existingSecret: for the GLPI application's DB credentials
  (needs MARIADB_DATABASE, MARIADB_USER, MARIADB_PASSWORD - no root
  password, preserving the existing least-privilege separation between
  the app-facing secret and the DB-server-facing secret). Used by
  php-fpm, all 5 init Jobs, and the cronjob. When set, glpi-secret.yaml
  is skipped. Applies whether mariadb.enabled is true or false.

Added glpi.mariadbSecretName / glpi.databaseSecretName helpers in
_helpers.tpl that resolve to the existingSecret name when set, or the
chart-managed Secret name otherwise. All secretRef consumers now go
through these helpers instead of hardcoding mariadb-glpi-secret /
glpi-secret.

Testing

• helm lint: 0 failures
• helm template (default): both glpi-secret and mariadb-glpi-secret created
• helm template --set mariadb.auth.existingSecret=doppler-mariadb-creds:
  mariadb-glpi-secret NOT created; StatefulSet + timezone Job reference
  doppler-mariadb-creds
• helm template --set glpi.database.existingSecret=doppler-glpi-db-creds:
  glpi-secret NOT created; all 8 consumers (php-fpm, nginx, 5 jobs,
  cronjob) reference doppler-glpi-db-creds

[eduardofraga]

1. Move files from kubernetes/glpi to helm/
2. Remove mariadb files and use sub-chart from helmforge.