Coder: mapping groupes Authentik → rôles Coder (RBAC)

## Contexte
Comme pour Omni/SAML (`omni-admin`/`omni-user`/`omni-viewer`), on veut que les rôles Coder soient dérivés automatiquement des groupes Authentik.

## À faire
- [ ] Définir la convention de groupes (ex: `coder-admin`, `coder-user`, `coder-auditor`)
- [ ] Configurer `CODER_OIDC_GROUP_FIELD` / `CODER_OIDC_GROUP_MAPPING` / `CODER_OIDC_USER_ROLE_MAPPING` dans les values de l'issue https://github.com/ClubCedille/k8s-shared/issues/320

## Dépend de
https://github.com/ClubCedille/k8s-shared/issues/318, https://github.com/ClubCedille/k8s-shared/issues/320

Start by reading dependent issues #318 and #320, especially the values configuration in issue #320, then verify how Coder receives Authentik OIDC groups. Define the group convention and configure CODER_OIDC_GROUP_FIELD, CODER_OIDC_GROUP_MAPPING, and CODER_OIDC_USER_ROLE_MAPPING; done means the configured groups produce the intended Coder roles.

Written by the indexing model from the issue text.