## Summary
This fixes an OIDC sign-in edge case where a stale `external_login_user`
record can still point to an organization or a deleted user.
In that situation, Gitea may keep resolving the external login to the
wrong account during sign-in. For affected instances, this matches the
behavior reported in #36439 and #37812, where a user signing in with
OIDC/Entra ID could appear as an organization, or hit a 404 after that
organization was removed.
## What changed
- validate the user resolved from `external_login_user` during
OAuth2/OIDC login
- ignore stale links when the linked user no longer exists
- ignore stale links when the linked user is not an individual user
- remove the stale external login row so the sign-in flow can relink the
external account to the correct user
## Related
- Fixes#37812
- Related to #36439
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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>