src: lib: stream: sink: image_sink: Rewrite to GStreamer-native pipeline

Replace the Rust image crate-based thumbnail pipeline with a fully
GStreamer-native approach using videoscale, videoconvert, and jpegenc.
This eliminates the need to decode to raw RGB, copy frame data out of
GStreamer, and re-encode in userspace.

Key changes:
- Decode -> videoscale -> capsfilter -> videoconvert -> jpegenc -> appsink
  replaces decode -> appsink -> image::thumbnail -> image::write_to
- JPEG quality and target resolution are updated dynamically per request
  by modifying jpegenc and capsfilter properties at runtime
- Software decoders limited to max-threads=1 to reduce CPU usage
- Pipeline runs at background priority via try_new_background()
- Decode only keyframes (skip-frame=non-key) for efficiency
- Harden decoders: disable QoS, reject corrupt output, throttle FKU
- Optimize appsink for low-latency single-frame capture
- Fix pipeline memory leak: set pipeline to Null after each capture
  so decoder drops reference frames and buffer pools are released
- malloc_trim(0) on Linux ensures glibc returns freed pages to OS

Author: joaoantoniocardoso