Fix: enforce authorization in template field endpoints

[rikled]

Assisted-by: Claude Code:Opus 4.8

• Target version: main

Summary

extractFields and fillFields resolved a global fileId via rootFolder->getFirstNodeById(), which searches across all users' storage. Any authenticated user could pass an arbitrary id to read another user's file contents or fill/overwrite it.
Changes:

• Resolve the id within the acting user's folder (getUserFolder()->getFirstNodeById()) so only accessible nodes (including valid shares) are returned, with a baseline READ check.
• Require PERMISSION_UPDATE before filling a PDF, since that overwrites the source in place.

Because resolution is now user-scoped, getPermissions() reflects the caller's share-level rights, making the permission check meaningful.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required