Add identity provider (SSO) support

## Problem
The dashboard currently has no support for external identity providers. Authentication (if any) is not integrated with common identity providers, making it hard to control access in a team/enterprise setting.

## Expected behavior
- Add support for at least one external identity provider (e.g. OAuth2/OIDC — Google, GitHub, or an OIDC provider like PingOne/PingFederate).
- Users can log into the dashboard via the configured identity provider instead of (or in addition to) any existing local auth.
- Configuration for the identity provider (client id/secret, issuer URL, etc.) should be handled via environment variables/config, not hardcoded.
- Document the setup steps for configuring a new identity provider.

## Notes
Start with a single provider (OIDC-generic or GitHub OAuth) to establish the pattern, then additional providers can be added incrementally.

The issue mentions adding OAuth2/OIDC support, likely to the dashboard's authentication flow. Look for existing auth code, perhaps in a `src/auth/` directory or similar. Research how to integrate a generic OIDC provider (like using `passport` or a similar library in TypeScript). The configuration should be via environment variables. Start by setting up a simple OIDC flow with a test provider, then adapt it for the dashboard. Check if there are any existing tests for authentication to understand the current setup.

Rédigé par le modèle d'indexation à partir du texte de l'issue.