fix(playback): force mp4 muxer; ffmpeg cant infer format from .tmp extension

Transcode wrote to {id}.mp4.tmp for atomic publish, but ffmpeg picks
the muxer from the output filename and rejected .tmp with Unable to
choose an output format. Add -f mp4 so the temp name is just storage
scratch, not a format hint.

backend/app/routers/photos.py

@@ -976,6 +976,10 @@ async def _transcode_to_h264_mp4(src: str, dst: str) -> bool:
                 '-c:a', 'aac',
                 '-b:a', '160k',
                 '-movflags', '+faststart',
+                # tmp filename ends in `.tmp`, which ffmpeg can't map to
+                # an output muxer — force the mp4 muxer explicitly so the
+                # name is just storage scratch, not a format hint.
+                '-f', 'mp4',
                 tmp,
             ],
             capture_output=True, text=True, timeout=600,