docs: clarify subtitle synchronization

[IronJKZ]

What changed

• presents automatic audio-based subtitle synchronization and manual adjustment in both feature lists
• separates end-user and contributor installation
• lists supported video, subtitle, and archive formats
• adds CI, Release, Python, and MIT badges
• aligns package description and keywords with the public positioning

Why

The implementation already synchronizes subtitle candidates with ffsubsync, rejects low-quality results, and supports manual whole-timeline adjustment, but the most visible repository information understated those capabilities.

Impact

Documentation, package metadata, and repository-contract tests only. No runtime behavior, version, release, dependency, CI, or security-setting change.

Validation

• complete pytest suite
• CLI and adjust help smoke tests
• sdist and wheel build
• tracked-path and Gitleaks checks
• git diff --check