Replace the Scribejava OAuth2 service with a custom implementation

[nroduit]

Description

OAuth2 / OpenID Connect authentication for DICOMweb and Viewer Hub used the Scribejava library. It brought its own HTTP client and thread pool, duplicated what the built-in JavaNetHttpClient already provides, and made it difficult to apply the request stall guard introduced for #899.

OAuth2ServiceFactory and the authorization-code / client-credentials flows are reimplemented on top of JavaNetHttpClient. Consequences:

• The scribejava 8.3.3 dependency and its IDEA external annotations are removed.
• The request stall guard is ported onto the new client: the request half of an exchange is bounded by time without progress (inactivityTimeout) instead of a whole-exchange deadline, so a large STOW-RS send on a slow link is only aborted when bytes stop flowing.
• The getConnectTimeout / getReadTimeout aliases deprecated in 4.7.3 are removed in favour of getConnectTimeoutMillis / getInactivityTimeoutMillis.
• Service invalidation on authentication changes (Implement service invalidation for updated authentication methods #896) is preserved.