refactor: consolidate format utils, extract route helpers, remove dead code
- Consolidate 5 duplicate format functions (formatDuration, formatRelativeTime,
formatFileSize, formatSubscriberCount) into shared utils/format.ts
- Extract parseIdParam() route helper, replacing 22 copy-paste blocks across 9 route files
- Remove dead exports: useScanStatus, useChannelContent (non-paginated),
getContentItemsByStatus, deleteQueueItem, deletePlaylistsByChannelId
- Fix as-any type assertion in system.ts (queueService already typed on FastifyInstance)
- Net: -411 lines, 23 files touched