OIDC naming is Dex-specific but works with any OIDC provider

Summary

The authentication system works great with any OIDC provider (we're using Keycloak), but the naming throughout the codebase is Dex-specific which is confusing.

Current Issues

1. Callback endpoint: /auth/dex/callback - hardcoded with "dex" in the path
2. Environment variables: DEX_OAUTH_CALLBACK_URL, DEX_OAUTH_CLIENT_ID, DEX_OAUTH_CLIENT_SECRET, DEX_ENABLED
3. Documentation: Focuses heavily on Dex setup rather than generic OIDC configuration

Suggested Changes

• Rename callback/login endpoint: /auth/dex/callback → /auth/oauth2/callback
• Rename env variables:
  • DEX_ENABLED → OIDC_ENABLED
  • DEX_OAUTH_CALLBACK_URL → OAUTH_CALLBACK_URL or OIDC_CALLBACK_URL
  • DEX_OAUTH_CLIENT_ID → OAUTH_CLIENT_ID or OIDC_CLIENT_ID
  • DEX_OAUTH_CLIENT_SECRET → OAUTH_CLIENT_SECRET or OIDC_CLIENT_SECRET
• Update docs to explain generic OIDC setup with Dex as one example

Why

Since ChaosCenter supports standard OIDC (works with Keycloak, Okta, Auth0, etc.), the naming should reflect that it's a generic OIDC integration, not Dex-specific. This would make it clearer for users that they can use any OIDC provider.

Additional Context

We successfully integrated ChaosCenter with Keycloak using the existing endpoints, but it was initially confusing whether Dex was required or just an example OIDC provider.