[Server] Full JWT Authorization Implementation in `reana-server`

[tomondre]

Build on the initial authorization proof-of-concept (#49) to implement a full JWT-based authorization layer in reana-server, including:

• Configurable trusted issuers and JWKS endpoints
• Support for full claim validation (e.g. sub)
• Secure error handling and status code responses
• Documentation for deployment and configuration
• Test coverage for supported scenarios and failure modes

This task represents the complete and production-ready implementation of backend JWT authorization.

[tomondre]

WIP 👀: reanahub/reana-server#741

[tomondre]

Requires review from the external team