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/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',
     },
   },