fix: TWO_HOURS constant in staging cleanup was actually 5 minutes

Cleanup deleted completed staging files as soon as 5 minutes had
passed instead of the intended 2 hours, so successfully downloaded
files could be silently removed before being moved out of staging.

src/qbt.js

@@ -468,7 +468,7 @@ function register(app) {
         let staged;
         try { staged = fs.readdirSync(SAVE_PATH); } catch { return; }
         const now = Date.now();
-        const TWO_HOURS = 5 * 60 * 1000;
+        const TWO_HOURS = 2 * 60 * 60 * 1000;
 
         const activeFiles = new Set(
             [...downloads.values()]