CLI: disable async crash-recovery backup in headless mode (fixes segfault)

QETProject schedules an asynchronous crash-recovery backup on construction
(writeBackup() -> QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)).
In one-shot CLI mode the QETProject is destroyed as soon as the command
returns, while that background write still references its m_backup_file
member — an intermittent use-after-free segfault during teardown (~1 in 6
runs; observed on --resave and --set-titleblock).

A crash-recovery backup is meaningless for a short-lived headless command,
so add QETProject::setBackupEnabled(false), called from the CLI entry in
main(). writeBackup() then early-returns, so no background write is ever
launched. Fixes the crash for all CLI commands. See #492.

sources/qetproject.h

@@ -105,6 +105,12 @@ class QETProject : public QObject
 		QVersionNumber declaredQElectroTechVersion();
 		void setTitle(const QString &);
 
+		/// Enable/disable the asynchronous crash-recovery backup for all
+		/// projects.  Disabled by the headless CLI: the backup write runs on a
+		/// background thread referencing the project, and a short-lived CLI
+		/// process can destroy the project before the write finishes (crash).
+		static void setBackupEnabled(bool enabled);
+
 			///DEFAULT PROPERTIES
 		BorderProperties defaultBorderProperties() const;
 		void             setDefaultBorderProperties(const BorderProperties &);
@@ -241,6 +247,8 @@ class QETProject : public QObject
 
 	// attributes
 	private:
+			/// When false, writeBackup() is a no-op (set by the headless CLI)
+		static bool m_backup_enabled;
 			/// File path this project is saved to
 		QString m_file_path;
 			/// Current state of the project