Fix download modal freezing by moving M3U save to completion only

autoSavePlaylistM3U was called on every 2-second poll cycle once any
track completed, flooding the server with heavyweight M3U generation
requests (fuzzy matching all tracks against the DB). This exhausted
Flask's thread pool, causing the batch status endpoint to hang and
killing the poller — making the modal freeze mid-download.

Now fires once when the batch completes instead of on every poll.

webui/static/script.js

@@ -15658,10 +15658,10 @@ function processModalStatusUpdate(playlistId, data) {
         document.getElementById(`download-progress-text-${playlistId}`).textContent = `${completedCount}/${missingCount} completed (${progressPercent.toFixed(0)}%)`;
         document.getElementById(`stat-downloaded-${playlistId}`).textContent = completedCount;
 
-        // Auto-save M3U file for playlists as downloads progress
-        if (completedCount > 0) {
-            autoSavePlaylistM3U(playlistId);
-        }
+        // Auto-save M3U file once when all downloads finish (not on every poll cycle).
+        // Previously this fired on EVERY 2-second poll when completedCount > 0, flooding
+        // the server with heavyweight M3U generation requests that exhausted Flask threads
+        // and caused the batch status endpoint to hang — killing the poller.
 
         // CLIENT-SIDE COMPLETION: Only complete when ALL task rows in the UI reflect a terminal state.
         // Using totalFinished (derived from DOM updates in THIS render pass) prevents premature
@@ -15678,6 +15678,11 @@ function processModalStatusUpdate(playlistId, data) {
             process.status = 'complete';
             updatePlaylistCardUI(playlistId);
 
+            // Save M3U once on completion (not during progress polling)
+            if (completedCount > 0) {
+                autoSavePlaylistM3U(playlistId);
+            }
+
             // Show the force download toggle again
             const forceToggleContainer = document.querySelector(`#force-download-all-${playlistId}`)?.closest('.force-download-toggle-container');
             if (forceToggleContainer) {