Improper authorization on OAuth sign-in callback silently re-enables administrator-disabled accounts

Affected: Go: code.gitea.io/gitea

### Summary

The OAuth2 sign-in callback in Gitea 1.26.1 unconditionally re-enables a locally-disabled account whenever the user authenticates through a linked external identity provider, silently undoing any administrator-initiated `Disable Account` action and issuing a fresh authenticated session in the same response. Once a user has linked any external IdP, the administrator's disable toggle becomes non-binding — the user can re-enable themselves on demand by completing one OAuth callback, regaining full read and write access to their repositories, organizations, and access tokens.

### Details

Improper Authorization is present on the `code` and `state` parameters of the `/user/oauth2/{source-name}/callback` endpoint in Gitea version 1.26.1. The handler `routers/web/auth/oauth.go::handleOAuth2SignIn` reads the local user's `IsActive` flag at lines 350-352 and, when it is `false`, sets `opts.IsActive = optional.Some(true)` before calling `user_service.UpdateUser` and immediately establishing a session. The active-state precondition that the request middleware `routers/web/web.go::verifyAuthWithOptions` enforces for every other request is therefore evaluated against the freshly-flipped row on the next request, so a site administrator's "disable user" action is silently undone the next time the user authenticates through any linked external identity provider.

The root cause is that the callback path treats the local `IsActive` flag as stale bookkeeping to reconcile against…

Ein entfernter, authentisierter Angreifer kann mehrere Schwachstellen in Gitea ausnutzen, um seine Privilegien zu erhöhen, Sicherheitsmaßnahmen zu umgehen, Daten zu manipulieren, vertrauliche Informationen offenzulegen oder einen Denial-of-Service-Zustand zu verursachen.