fix(auth): ignore stale OIDC external login links to organizations (#37875)

## 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

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

models/user/external_login_user.go

@@ -80,8 +80,11 @@ func init() {
 }
 
 // GetExternalLogin checks if a externalID in loginSourceID scope already exists
-func GetExternalLogin(ctx context.Context, externalLoginUser *ExternalLoginUser) (bool, error) {
-	return db.GetEngine(ctx).Get(externalLoginUser)
+func GetExternalLogin(ctx context.Context, loginSourceID int64, externalID string) (*ExternalLoginUser, bool, error) {
+	return db.Get[ExternalLoginUser](ctx, builder.Eq{
+		"external_id":     externalID,
+		"login_source_id": loginSourceID,
+	})
 }
 
 // LinkExternalToUser link the external user to the user
@@ -118,6 +121,12 @@ func RemoveAllAccountLinks(ctx context.Context, user *User) error {
 	return err
 }
 
+// RemoveExternalLoginByExternalID removes a specific external login link by its provider-side identifier.
+func RemoveExternalLoginByExternalID(ctx context.Context, loginSourceID int64, externalID string) error {
+	_, err := db.GetEngine(ctx).Where("external_id=? AND login_source_id=?", externalID, loginSourceID).Delete(new(ExternalLoginUser))
+	return err
+}
+
 // GetUserIDByExternalUserID get user id according to provider and userID
 func GetUserIDByExternalUserID(ctx context.Context, provider, userID string) (int64, error) {
 	var id int64