feat: playlists command for bulk playlist operations

[jdwit]

Summary

• Adds ytstudio playlists sub-app with 9 commands: list, show, create, update, delete, items, add, remove, reorder.
• Targets the playlist surface YouTube Studio handles poorly at scale (bulk add by search, view-based reorder).
• All writes are dry-run by default; --execute applies. Quota counters and partial-progress reporting on every bulk loop.

Highlights

• Power-user one-liners:
  • ytstudio playlists add PL... --from-search "shorts compilation" -n 20 --execute
  • ytstudio playlists reorder PL... --by views --execute
• Update re-specifies current snippet fields to avoid the silent-nuke trap.
• Uploads playlists (UU...) are refused early for any mutating command.
• Specific errors mapped: manualSortRequired, playlistContainsMaximumNumberOfVideos, videoNotFound, quotaExceeded.

Test plan

• uv run ruff check .
• uv run ruff format --check .
• uv run pytest -q (238 passed)
• Smoke against a real channel: list, show, create, add by search, reorder, delete.

Out of scope

• YAML manifest sync (playlists sync FILE.yaml); follow-up PR.