Admin UI credential re-seed from credlist files

[tire-fire]

Currently, credlist files are only read at initial database seed time. If a credlist file is modified after seeding (e.g., removing accounts, fixing usernames, or adding new users), the only way to reflect those changes in the database is to manually delete rows from the DB and restart the server, or wipe the entire database.

Add a button in the admin UI (e.g., on the credlists/PCR management page) that allows an administrator to re-seed credentials from the current credlist files on disk.

Use case: During competition setup, organizers iterate on credlists (removing default accounts, adding service accounts, fixing typos). Currently this requires direct database manipulation, which is error-prone and disruptive.