refactoring shit so this shit works

[donutinit]

this was lazyly vibe coded with claude i dont even know what "c" is

anyway it actually fixes some real stuff:

• bitrate slider was ignored (encoder got built before the settings were read) -> now the selected kbps is actually applied
• mp4 aac had no esds / AudioSpecificConfig, so it showed as mp4a.40.0 / profile=-1 and played SILENT in firefox + safari. now the encoder's asc is published as the 'esds' magic cookie -> proper mp4a.40.2 (aac-lc). this fixes the browser-silence issue people reported
• audio packets now flagged as keyframes (sync samples) so seeking is correct
• cleaned up the 3x copy-pasted encode/drain code, fixed a host-buffer lock leak on encode error, and started checking ffmpeg return values
• makefile pulls ffmpeg headers via pkg-config so it builds on fedora (headers live in /usr/include/ffmpeg, not /usr/include)
• added LICENSE (gplv3, matches the readme) + fedora/rhel build deps in the readme

verified by muxing real packets to mp4: ffprobe reports aac (LC) / mp4a and the audio decodes non-silent.