Updates from 2025-09-28 (de03c395)

Added thumbnails support for image items, generating and serving via the new /thumbs/ endpoint, which are shown in listing pages to reduce transfer size and loading times.
Added pagination to user profile and collections pages.
Added a listing of instance configuration flags to the admin panel.
Added an option to clear all BAK files to the admin panel
Added a dynamically-shown link to the admin panel in the footer
Fixed parsing of some instance configuration keys.
Fixed a bug with user handling in the API endpoints, where the inexistent Flask session user would try to be accessed, causing a runtime error.