Merge pull request #27864 from peters:patch-2

Add OPENCV_FFMPEG_SKIP_LOG_CALLBACK to preserve custom FFmpeg logging #27864 
 
OpenCV’s InternalFFMpegRegister overwrites av_log_set_callback, blocking custom FFmpeg log handlers in statically linked apps.

Add `OPENCV_FFMPEG_SKIP_LOG_CALLBACK` to let applications keep their own logging.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

doc/tutorials/introduction/env_reference/env_reference.markdown

@@ -273,6 +273,7 @@ Some external dependencies can be detached into a dynamic library, which will be
 | OPENCV_FFMPEG_THREADS | num | | set FFmpeg thread count |
 | OPENCV_FFMPEG_DEBUG | bool | false | enable logging messages from FFmpeg |
 | OPENCV_FFMPEG_LOGLEVEL | num | | set FFmpeg logging level |
+| OPENCV_FFMPEG_SKIP_LOG_CALLBACK | bool | false | do not install OpenCV's FFmpeg log callback (preserve default/user callback) |
 | OPENCV_FFMPEG_DLL_DIR | dir path | | directory with FFmpeg plugin (legacy) |
 | OPENCV_FFMPEG_IS_THREAD_SAFE | bool | false | enabling this option will turn off thread safety locks in the FFmpeg backend (use only if you are sure FFmpeg is built with threading support, tested on Linux) |
 | OPENCV_FFMPEG_READ_ATTEMPTS | num | 4096 | number of failed `av_read_frame` attempts before failing read procedure |