perf(id3,cli): parallelise cover art processing with encoding

- Start cover art processing concurrently in separate goroutine before encoding
- Collect pre-processed artwork bytes after encoding completes
- Add CoverArtData field to TagInfo for pre-processed cover art bytes
- Refactor addCoverArt to delegate to addCoverArtData when data available
- Hides cover art processing latency (~200-500ms) behind encoding time
- Maintains backward compatibility with CoverArtPath fallback

This optimization improves perceived performance for short podcast encoding
jobs (< 1 minute) where cover art processing previously blocked on critical path.

deps: update ffmpeg-statigo to v8.0.1.0

fix(demo): swap file argument order in demo commands

- Corrects argument order in vhs tape demo from `./jivedrop file.md file.flac` to `./jivedrop file.flac file.md`
- Updates sleep timer in tape from 10s to 4s for smoother demo
- Aligns the `mp3` test command in justfile with the same file arguments
- Adds a descriptive comment for the vhs target in justfile

docs: update tagline and description to be more concise

- Streamlines the project description in the README and CLI help text
- Makes the value proposition clearer with "Drop the mix, ship the show" tagline
- Maintains the same core message about MP3 conversion with metadata and artwork

fix(workflow): refactor GitHub release workflow for parallel builds

- Split single build job into separate jobs for better organization and parallelism
- Create a dedicated job for release creation separate from build tasks
- Organize build jobs by platform (Linux/macOS) with matrix for architectures
- Replace wildcard patterns with explicit file paths to prevent uploading unintended files
- Improve variable references and build environment configurations
- Ensure assets upload to the same release using job dependencies