[codex] Add Docker OPDS service

[varo6]

Summary

• Add a Docker-only OPDS 1.2 catalog for mounted CBZ libraries.
• Convert CBZ files to XTC/XTCH on demand with a persistent cache.
• Add server-side image processing via sharp while keeping the existing browser converter unchanged.
• Document Docker usage and the current CrossPoint compatibility caveat.

Fixes #30

Validation

• bun test
• bun run build
• docker compose build
• Local Bun smoke test for /opds, /opds/books, and XTC download
• Built Docker image smoke test with mounted temporary CBZ library