fix: optimize FFmpeg encoding to reduce memory usage

- Switch from VP9 to VP8 codec for WebM (less memory-intensive)
- Add realtime encoding deadline and fast CPU preset
- Add ultrafast preset for x264 encoding
- Set explicit bitrates to control memory usage
- Use libvorbis instead of libopus for better compatibility

This fixes "memory access out of bounds" errors during video conversion
in browser environments with limited WASM memory.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/converters/ffmpegService.ts

@@ -120,12 +120,24 @@ function buildFFmpegArgs(
   // Format-specific settings
   switch (outputFormat) {
     case 'webm':
-      if (!options.videoCodec) args.push('-c:v', 'libvpx-vp9');
-      if (!options.audioCodec) args.push('-c:a', 'libopus');
+      // Use VP8 by default (less memory-intensive than VP9)
+      if (!options.videoCodec) args.push('-c:v', 'libvpx');
+      if (!options.audioCodec) args.push('-c:a', 'libvorbis');
+      // Optimize for faster encoding and lower memory usage
+      args.push('-deadline', 'realtime');
+      args.push('-cpu-used', '8');
+      // Set quality/bitrate if not specified
+      if (!options.videoBitrate) args.push('-b:v', '1M');
+      if (!options.audioBitrate) args.push('-b:a', '128k');
       break;
     case 'mp4':
       if (!options.videoCodec) args.push('-c:v', 'libx264');
       if (!options.audioCodec) args.push('-c:a', 'aac');
+      // Use faster preset for browser encoding
+      args.push('-preset', 'ultrafast');
+      args.push('-tune', 'zerolatency');
+      if (!options.videoBitrate) args.push('-b:v', '1M');
+      if (!options.audioBitrate) args.push('-b:a', '128k');
       break;
     case 'mp3':
       if (!options.audioCodec) args.push('-c:a', 'libmp3lame');

lib/utils/formatMappings.ts

@@ -218,14 +218,14 @@ export const FORMAT_PRESETS: FormatPreset[] = [
   {
     id: 'web-video',
     name: 'Web Video',
-    description: 'Optimize video for web playback',
+    description: 'Optimize video for web playback (VP8 for better compatibility)',
     category: 'video',
     sourceFormats: ['mp4', 'avi', 'mov', 'mkv'],
     targetFormat: 'webm',
     options: {
-      videoCodec: 'libvpx-vp9',
+      videoCodec: 'libvpx',
       videoBitrate: '1M',
-      audioCodec: 'libopus',
+      audioCodec: 'libvorbis',
       audioBitrate: '128k',
     },
   },