Playlist IDs refactor

[semohr]

This pull request refactors how playlist identifiers are handled across the codebase, moving from service-specific PlaylistIDs dictionaries to strongly-typed, immutable PlaylistID dataclasses for each service (e.g., Plex and Spotify). This change improves type safety, clarity, and extensibility for playlist identification and lookup. The update also removes now-obsolete ID extraction utilities and ensures all playlist operations use the new identifier types.

Todos:

• Decide how to handle offline playlists:
  • Either we add a OfflinePlaylistID as wrapper for multiple other ids
  • or we allow a list of ids (makes working within our abstraction a bit awkward tho)
• Fix playlist tests base for new Id system
• Changelog
• Notebooks
• Add tests for OfflinePlaylistID

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

❌ Patch coverage is 73.76426% with 69 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
plistsync/services/spotify/library.py	15.00%	17 Missing ⚠️
plistsync/services/tidal/library.py	19.04%	16 Missing and 1 partial ⚠️
plistsync/services/plex/library.py	47.05%	6 Missing and 3 partials ⚠️
plistsync/core/playlist.py	84.44%	4 Missing and 3 partials ⚠️
plistsync/services/plex/playlist.py	84.37%	5 Missing ⚠️
plistsync/services/spotify/playlist.py	86.20%	4 Missing ⚠️
plistsync/services/traktor/playlist.py	88.57%	4 Missing ⚠️
plistsync/services/tidal/playlist.py	89.28%	3 Missing ⚠️
plistsync/services/traktor/library.py	86.95%	1 Missing and 2 partials ⚠️

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