Merge videos

Merge multiple media files.

POST /merge
Content-Type: application/json

{
  "fileIds": ["string", "string"],
  "outputFormat": "string",
  "mode": "replace" | "append"
}

Uses the first file (fileIds[0]) as the base, and swaps it with the results
Mode defaults to append
Automatically detects and matches resolution
Preserves aspect ratio with padding
Uses complex filter chain for proper concatenation
Output encoding:
- Video: H.264 (libx264) with fast preset, CRF 22
- Audio: AAC at 192kbps