[FEAT] Add optional user auth backend

[NeonDaniel]

Objective

Implement some optional authentication methods for HTTP endpoint access. Consider offering more requests for logged in users and possibly requiring real authorized user accounts for access to Neon endpoints.

Initial Implementation requirements

• Allow minimal option with a single username and password (hash?) from config
• Allow for login via external oauth provider

Other Considerations

• Eventually, we may require some kind of user login for Neon applications (i.e. no default free access)
• Any implementation here should be optional for self-hosted instances that don't need the added complexity
• Rate limits might be adjusted for different routes

[NeonDaniel]

The authentication here may also define different levels of skill access per-user

[NeonDaniel]

One option here is to hook into the klatapi endpoint to authenticate with that existing user db. We can also extend those profiles to store additional user params for settings/authorization

[NeonDaniel]

Some specific areas that may be individually permissioned:

• assist endpoints
• backend endpoints
• llm endpoints (consider split from backend)
• node endpoints (currently the only endpoints behind any auth)

Related to #31: Exposing other services through HANA would allow for unified permissions management