feat: 支持 MJPEG 解码与 MSD 目录配置

- FFmpeg/hwcodec 增加 RKMPP MJPEG 解码与 RAM FFI，ARM 构建启用对应解码器
  - 共享视频管线新增 MJPEG 解码路径（RKMPP/TurboJPEG），优化 WebRTC 发送与 MJPEG 去重
  - MSD 配置改为 msd_dir 并自动创建子目录，接口与前端设置同步更新
  - 更新包依赖与版本号

build/cross/Dockerfile.armv7

@@ -206,6 +206,9 @@ RUN mkdir -p /tmp/ffmpeg-build && cd /tmp/ffmpeg-build \
         -Dlibrga_demo=false \
     && ninja -C build \
     && ninja -C build install \
+    # Create static librga.a from built objects (rkrga uses shared_library)
+    && ar rcs /usr/arm-linux-gnueabihf/lib/librga.a $(find build -name '*.o') \
+    && ranlib /usr/arm-linux-gnueabihf/lib/librga.a \
     && sed -i 's/^Libs:.*$/& -lstdc++ -lm -lpthread/' /usr/arm-linux-gnueabihf/lib/pkgconfig/librga.pc \
     && cd .. \
     # Create pkg-config wrapper for cross-compilation
@@ -256,8 +259,10 @@ RUN mkdir -p /tmp/ffmpeg-build && cd /tmp/ffmpeg-build \
         --disable-avfilter \
         --disable-avdevice \
         --disable-postproc \
-        # Disable all decoders
+        # Disable all decoders (re-enable only what we need)
         --disable-decoders \
+        --enable-decoder=mjpeg \
+        --enable-decoder=mjpeg_rkmpp \
         # Disable all encoders, enable only needed ones
         --disable-encoders \
         --enable-encoder=h264_rkmpp \
@@ -281,8 +286,7 @@ RUN mkdir -p /tmp/ffmpeg-build && cd /tmp/ffmpeg-build \
         --disable-bsfs \
         --enable-bsf=h264_mp4toannexb \
         --enable-bsf=hevc_mp4toannexb \
-        # Disable hardware decoding
-        --disable-hwaccels \
+        # Hardware decoding uses explicit rkmpp decoder (no hwaccel flag)
         # Disable other unused features
         --disable-indevs \
         --disable-outdevs \