feat: Refresh token during active user sessions

[solracsf]

Resolves #1384

How it works end-to-end:

• Token lifetime is e.g. 300s. After 150s (isExpiring() threshold), the next Nextcloud request triggers a proactive refresh_token call to IdP.

  user_oidc/lib/Model/Token.php

  Line 77 in 93e7593

  return time() > ($this->createdAt + (int)($this->expiresIn / 2));

• IdP resets its SSO session idle timer on that token endpoint call.
• The existing locking mechanism ensures concurrent requests don't hammer the IdP (only one refresh happens per session, others wait and reuse the result).
• No new settings needed: the behavior is gated entirely on the existing store_login_token setting and a non-null refresh token.

No breaking changes to the public API, no behavior change for non-OIDC sessions, and no behavior change when store_login_token is disabled.

[julien-nc]

Nice!