Tinyauth: Unauthenticated login attempts can trigger global login lockdown denial of service

### Summary Tinyauth's login rate-limit bookkeeping can enter a global lockdown mode when its in-memory login-attempt map reaches 256 distinct identifiers. Because unauthenticated `POST /api/user/login` requests for unknown usernames are recorded in this same map, a remote unauthenticated attacker can submit 257 unique bogus usernames and cause valid credentials for unrelated users to be treated as locked until `auth.loginTimeout` expires. This was confirmed against the stable `v5.0.7` release. With default configuration, `auth.loginTimeout` is 300 seconds and `auth.loginMaxRetries` is 3, so the denial lasts about 5 minutes and can be repeated. ### Details In stable `v5.0.7`, the login endpoint is registered at `internal/controller/user_controller.go:45` and accepts unauthenticated JSON credentials in `loginHandler` at `internal/controller/user_controller.go:50`. Before validating credentials, it calls `controller.auth.IsAccountLocked(req.Username)` at `internal/controller/user_controller.go:65`. When a username does not exist, the login handler records a failed login attempt for the attacker-controlled username with `controller.auth.RecordLoginAttempt(req.Username, false)` at `internal/controller/user_controller.go:83`. Invalid passwords for existing users do the same at `internal/controller/user_controller.go:94`. The rate-limit map has a hard cap of 256 records at `internal/service/auth_service.go:29`. `RecordLoginAttempt` checks `len(auth.loginAttempts) >= MaxLoginAttempt