fix: treat email addresses case-insensitively (#37600)

Fixes #36184 and three more discovered cases.

---
This PR was written with the help of Claude Opus 4.7

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>

models/asymkey/ssh_key_principals.go

@@ -40,7 +40,7 @@ func CheckPrincipalKeyString(ctx context.Context, user *user_model.User, content
 				if !email.IsActivated {
 					continue
 				}
-				if content == email.Email {
+				if strings.EqualFold(content, email.LowerEmail) {
 					return content, nil
 				}
 			}