Add media storage pipeline

[Eoic]

Summary

• Add authenticated media asset storage backed by server disk
• Add upload, download, delete, and usage endpoints with quota enforcement
• Add synced book media references and validation for uploaded media IDs
• Delete owned media assets when synced books are deleted

Client support

• Add client media upload and cache pipeline client#16

Verification

• .venv/bin/ruff check papyrus tests
• .venv/bin/pytest -q