OAuth2 / OIDC On-Behalf-Of (OBO) Dynamic Token Exchange

[spelech]

Implemented OAuth2 / OIDC On-Behalf-Of (OBO) token exchange (RFC 8693) via TokenExchangeSecretRetriever to dynamically bridge static client AppKeys to dynamic short-lived JWTs for downstream MCP servers. Added support for PocketID, Azure AD, Okta, and generic OIDC providers, including token caching and UI settings.

Fixes #102

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PR created automatically by Jules for task 6942985795953965040 started by @spelech

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[spelech]

Merged manually