Add client media upload and cache pipeline

[Eoic]

Summary

• Add authenticated media API methods for usage, upload, download, and delete
• Add book file/cover media IDs to the local sync model and upload mapper
• Queue media uploads after signed-in imports and retry after sync/upload completion
• Show real server file-storage usage in Storage & sync
• Lazy-load private uploaded covers and cache downloaded book files in platform-local storage

Depends on

• Add media storage pipeline server#3

Verification

• flutter analyze
• flutter test