SeekQuote Web v0.1.15

- Server-side FFmpeg conversion constrained to one worker thread for shared hosting.
- libx264 explicitly uses threads=1 and lookahead_threads=1.
- Uses the veryfast x264 preset to reduce server CPU load.
- Keeps the known-good plugin target: H.264 High, 30 fps, yuv420p, 2.5 Mbps,
  keyframes every 29 frames, 1/600 track timescale, AAC-LC 44.1 kHz stereo.
- Improved FFmpeg failure details if the host blocks another feature.
