feat: login gates and the login event

Register publishes a *classes.LoginGates registry. POST /login consults
it through controllers.CheckLoginGates after the password, activation and
throttle checks and the bcrypt rehash, and before the restore of a
deactivated account, last_seen and the mint: a gate result replaces the
token response, a gate error answers the opaque 500. The restore and its
reactivate mail now run after the gates, so a password alone cannot
reactivate an account a second factor protects. Without a gate /login
answers as before; refresh never consults the gates.

classes.LoginEvent (golem15.user.login) fires after every password login
that minted a token; controllers.FireLoginEvent lets other plugins fire it.

classes/events.go

@@ -25,6 +25,10 @@ const BackendUnlinkedEventName = "user.BackendUnlinked"
 // AccessRevokedEventName is the festival name of AccessRevokedEvent.
 const AccessRevokedEventName = "user.AccessRevoked"
 
+// LoginEventName is the festival name of LoginEvent, the PHP plugin's
+// golem15.user.login.
+const LoginEventName = "golem15.user.login"
+
 // GetApiArrayEvent is the fire-and-collect payload other plugins extend.
 type GetApiArrayEvent struct {
 	User *models.User
@@ -111,3 +115,14 @@ type AccessRevokedEvent struct {
 	Reason  string
 	Tx      *gorm.DB
 }
+
+// LoginEvent fires after a sign-in issued a session token: a password login,
+// a sign-in completed by a second factor, a passkey sign-in or a device
+// hand-over (Source is one of the LoginSource constants). It does not fire
+// on refresh, on /register or for a sign-in a login gate held back. It fires
+// after the token was minted and outside any transaction; a listener error
+// is logged and never fails the sign-in (PHP's golem15.user.login).
+type LoginEvent struct {
+	User   *models.User
+	Source string
+}