fix(media): encode gif-to-video WebM as yuv420p

GIFs decode to bgra/gbrap (alpha); libvpx-vp9 rejects those pixel formats so the encoder never opened (ffmpeg exit 234), breaking GIF->WebM for essentially every GIF. Flatten to yuv420p in the webm branch, matching what the mp4 branch already does.

apps/api/src/routes/tools/gif-to-video.ts

@@ -34,6 +34,10 @@ export function registerGifToVideo(app: FastifyInstance) {
             "0",
             "-crf",
             "32",
+            // GIFs decode to bgra/gbrap (alpha); libvpx-vp9 rejects those pixel
+            // formats, so flatten to yuv420p just like the mp4 branch does.
+            "-pix_fmt",
+            "yuv420p",
             "-an",
             out,
           ];