Fix following past 48 entries and make the e621 page size configurable

Follow lists were paginated at the API default of 48, but the SPA treats
them as complete sets: the tag/pool toggles read their state from page one
(so the 49th follow looked unfollowed and its spinner waited for a page that
could never contain it) and the Followed page rendered only 48 cards while
showing that as the count. Both follow endpoints are now unpaginated — they
are per-user sets and still restricted to the caller's rows — and the three
consumers take plain arrays.

Post visibility: the old J621-Django online view fetched limit=320 (e621's
maximum) while ours hard-coded 48, and fetchPostsByIds capped id batches at
100. The Online browser now has a 'Posts per page' setting (48/100/200/320)
in its sidebar, mirrored in Account -> Browsing preferences, stored per user
as e621_per_page and also used for pool loading; the id-batch cap is raised
to 320.

Tests: follow list shape/isolation (4) and preference validation/merge (3)
added; the full backend suite is 46 green. Live-checked the array response
shape and the preference bounds (200 accepted, 500 rejected).

ROADMAP.md

@@ -145,7 +145,9 @@ Files now stage first and are resolved before entering the library.
 - [x] Profile pictures: staff Users page and a self-service Account picker
       (searchable library grid, remove supported) set avatars from library J-IDs
 - [x] Profile extras: per-user landing page, default rating filter/sort, items
-      per page and thumbnail size (synced to the account, applied on load)
+      per page and thumbnail size (synced to the account, applied on load),
+      plus e621 posts per page (48/100/200/320) for the Online browser and
+      pool loading
 - [x] Backend-less local mode: with no backend connected the SPA runs on the
       e621-facing pages only (Online, Pools) using credentials stored in the
       browser, and the shell offers a "Setup Backend" button instead