feat: Generate video covers with bounded ffmpeg processing

[PerovXP]

src issue: #28

bot now generates a JPEG cover when sending videos

if extraction fails / the queue is full, sending can continue without a cover

• upgrade mtcute to 0.32.1; add ffmpeg to the docker image and local setup requirements
• extract covers from a seekable temporary file and remove it afterward
• limit processing to 2 ffmpeg processes and 8 queued requests; a running process will be terminated when the deadline (10s) expires