Investigate approaches for adding OAuth2/OIDC support

## Goal

Investigate possible approaches to add OAuth2 and/or OIDC authentication support to Graph Explorer. Determine how authentication flows would integrate with the existing proxy server architecture, what libraries or frameworks are best suited, and what trade-offs exist between different approaches.

Key questions to answer:
- Which OAuth2/OIDC flows are appropriate for Graph Explorer (e.g. Authorization Code with PKCE, Client Credentials)?
- How would token management work given the client-side architecture and backend proxy?
- What identity providers should be targeted (e.g. Amazon Cognito, Okta, Azure AD, generic OIDC)?
- How would authenticated requests be forwarded to the graph database?
- What changes are needed in the proxy server vs. the client?

## Expected Outcome

- A written recommendation documenting the viable approaches with pros and cons
- A rough estimate of effort for the recommended approach
- A tasked-out epic if the spike confirms feasibility

## Related Issues

_None_

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