prioritize moov atom (last 10MB) + header for MP4/MOV streaming

Browsers need the moov atom (at end of file) to start playback.
Mark these pieces as critical so WebTorrent downloads them first,
alongside the file header at the beginning.

Author: ralyodio

src/lib/streaming/streaming.ts

@@ -776,6 +776,50 @@ export class StreamingService {
     file.select();
     logger.debug('File selected for download priority');
 
+    // For MP4/MOV files, prioritize downloading the last ~10MB where the moov atom lives
+    // Without the moov atom, browsers cannot begin playback (even for raw streaming)
+    const ext = file.name.split('.').pop()?.toLowerCase();
+    const MOOV_FORMATS = new Set(['mp4', 'm4v', 'mov', 'm4a', '3gp', '3g2']);
+    if (MOOV_FORMATS.has(ext ?? '') && file.length > 10 * 1024 * 1024) {
+      const fileOffset = (file as unknown as { offset: number }).offset ?? 0;
+      const moovBytes = 10 * 1024 * 1024; // 10MB should cover any moov atom
+      const moovStart = fileOffset + file.length - moovBytes;
+      const moovStartPiece = Math.floor(moovStart / torrent.pieceLength);
+      const moovEndPiece = Math.ceil((fileOffset + file.length) / torrent.pieceLength) - 1;
+      
+      // Also prioritize the first few pieces for the ftyp header
+      const headerEndPiece = Math.min(
+        Math.ceil((fileOffset + 1 * 1024 * 1024) / torrent.pieceLength),
+        torrent.pieces.length - 1
+      );
+      const headerStartPiece = Math.floor(fileOffset / torrent.pieceLength);
+      
+      logger.info('Prioritizing moov atom + header for MP4/MOV', {
+        fileName: file.name,
+        fileSizeMB: (file.length / (1024 * 1024)).toFixed(0),
+        headerPieces: `${headerStartPiece}-${headerEndPiece}`,
+        moovPieces: `${moovStartPiece}-${moovEndPiece}`,
+        totalPieces: torrent.pieces.length,
+        pieceLength: torrent.pieceLength,
+      });
+      
+      // Use select with high priority for these critical ranges
+      // WebTorrent select(start, end, priority) — higher priority = downloaded first
+      file.select();
+      try {
+        // Critical pieces for the header (beginning of file)
+        for (let i = headerStartPiece; i <= headerEndPiece; i++) {
+          (torrent as unknown as { critical: (start: number, end: number) => void }).critical(i, i);
+        }
+        // Critical pieces for moov atom (end of file)
+        for (let i = moovStartPiece; i <= moovEndPiece; i++) {
+          (torrent as unknown as { critical: (start: number, end: number) => void }).critical(i, i);
+        }
+      } catch (err) {
+        logger.warn('Could not set critical pieces', { error: String(err) });
+      }
+    }
+
     // Log current download state
     const fileProgress = (file as unknown as { progress: number }).progress ?? 0;
     const downloadedBytes = fileProgress * file.length;