Prebuilt patched qtmultimedia FFmpeg plugin (Windows, AV1 fix)

Not a Vivace release -- a build-infrastructure artifact consumed by
release.yml/nightly.yml's CI workflow.

Contains ffmpegmediaplugin.dll (built from Qt 6.11.1's qtmultimedia with
patches/qtmultimedia-av1-hwaccel-disable.patch applied) plus the matching
dav1d-enabled FFmpeg DLLs it needs (avcodec-61.dll, avformat-61.dll,
avutil-59.dll, swresample-5.dll, swscale-8.dll -- soname-matched to Qt's
own officially bundled FFmpeg, built from BtbN/FFmpeg-Builds' n7.1 line).

Together these fix AV1 video decoding, which the official Qt FFmpeg
backend cannot do reliably -- see the "AV1 support" section in README.md
for the full explanation.

Rebuilt locally against a Qt 6.11.1 msvc2022_64 kit; update this release's
asset whenever the patch or Qt version changes.