fix: page Plex library scans

[n2b8]

Summary

• page Plex library section fetches instead of requesting one huge /library/sections/:id/all response
• raise fast-xml-parser entity-expansion limits for trusted Plex payloads while keeping entity processing enabled
• avoid large-library sync failures like Entity expansion limit exceeded: 1001 > 1000

Verification

• built server and client locally
• exercised Plex library pagination with a mocked smoke test
• locally tested against a real library that previously failed during scan