feat: issue per-user preauth keys for shared Headscale

[0x7fffff92]

Summary

• Verify X-Authorization through the Olares token verification service.
• Use the verified Olares username instead of the fixed default Headscale user.
• Create the corresponding Headscale user when it does not exist.
• Handle concurrent user creation by re-reading the user after creation.
• Issue reusable preauth keys with a 24-hour lifetime.
• Build the complete Go package so the identity verification code is included.

Motivation

The shared Headscale deployment serves multiple Olares users. Preauth keys must therefore be issued for the authenticated user's corresponding Headscale account rather than a global default account.

Requests with a missing or invalid Olares session token are rejected with 401 Unauthorized.