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Issue Title: Support running server behind a subdirectory with backward compatibility for Socket.IO and OIDC

Issue Title: Support running server behind a subdirectory with backward compatibility for Socket.IO and OIDC: a task in LegoFlow-SWE (Harbor dataset). Description We want to deploy the server behind a reverse proxy that serves it under a subdirectory (e.g., /audiobookshelf ). Several changes are…

The task

**Description** We want to deploy the server behind a reverse proxy that serves it under a subdirectory (e.g., `/audiobookshelf`). Several changes are needed to avoid breaking existing clients and configurations.

Part of Lego-X/LegoFlow-SWE.