Securely wipe state.db on exit (#216)

- Add secure_wipe_file() to utils/crypto.py for file overwrites + deletion
- Add secure_wipe parameter to StateManager.stop() method
- Wipes all SQLite WAL files (.db, .db-wal, .db-shm) with 3 passes
- Normal exit now calls state.stop(secure_wipe=True) to clear state database
- Falls back to regular delete if secure wipe fails
- Prevents state.db from persisting on disk after shutdown

bigbrother.py

@@ -244,7 +244,7 @@ def start(daemon, passive_only):
     res_mon.stop()
     engine.stop_all()
     tool_manager.stop_all()
-    state.stop()
+    state.stop(secure_wipe=True)
     bus.stop()
     console.print("[green]SystemMonitor stopped.[/green]")