Add NoOp email service, /docs shortcut and fix streak timezone bug

Replace nil email service pattern with NoOpEmailService (Null Object)
to eliminate nil pointer panics across all handlers.

Add /docs route as a shortcut to Swagger UI.

Fix streak calculation returning 0 when server timezone differs from
UTC — CreatedAt was not converted to UTC before date extraction.

internal/domain/services/email_service.go

@@ -10,4 +10,11 @@ type EmailMessage struct {
 type EmailService interface {
 	Send(message EmailMessage) error
 	HealthCheck() error
+	IsEnabled() bool
 }
+
+type NoOpEmailService struct{}
+
+func (n *NoOpEmailService) Send(_ EmailMessage) error { return nil }
+func (n *NoOpEmailService) HealthCheck() error        { return nil }
+func (n *NoOpEmailService) IsEnabled() bool           { return false }