feat(library): add maintenance and status commands

[dogbertdev]

Summary

• add a new library command namespace and move section listing under library list
• implement library update [section|all] and library clean [section|all]
• wire library maintenance client methods for refresh, trash cleanup, bundle cleanup, cache deletion, and DB optimization
• add library status to surface active /activities and /status/sessions/background tasks from Plex
• update README examples for the new library command surface

Commits

• feat(library): add list update and clean commands
• feat(library): add status command for active tasks

Testing

• go test ./...
• pre-push hook checks passed (format/lint/tests)

Notes

• No repository PR template was found under .github/.
• npm run preflight is not applicable in this Go repo (no package.json).