[Feature]: Add OAuth login provider support for Google, GitHub, Facebook, and future providers.

[Bccorb]

Problem statement

Adopters want variety in their login preferences. Include Oauth logins as a feature and mechanic that can be opted into.

Proposed solution

• Providers can be configured and enabled independently.
• OAuth start and callback endpoints validate state and nonce.
• Provider identities can link to users according to explicit policy.
• Provider tokens are not leaked in responses, logs, or auth events.
• Tests cover successful login, denied callback, and linking conflicts.

Alternatives considered

No response

Impact area

Developer experience