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).

frontend/src/features/pools/PoolDetailPage.tsx

@@ -25,17 +25,18 @@ import { useIsMobile } from "@/lib/useMediaQuery";
 import { useE621 } from "@/store/e621";
 import { useUi } from "@/store/ui";
 
-const CHUNK = 100;
+const CHUNK_FALLBACK = 100;
 
 async function fetchPostsChunked(
   credentials: ReturnType<typeof effectiveCredentials>,
   ids: number[],
+  chunk: number,
   signal?: AbortSignal,
 ): Promise<E621Post[]> {
   const posts: E621Post[] = [];
-  for (let index = 0; index < ids.length; index += CHUNK) {
-    const chunk = ids.slice(index, index + CHUNK);
-    posts.push(...(await fetchPostsByIds(credentials, chunk, signal)));
+  for (let index = 0; index < ids.length; index += chunk) {
+    const slice = ids.slice(index, index + chunk);
+    posts.push(...(await fetchPostsByIds(credentials, slice, signal)));
   }
   return posts;
 }
@@ -54,8 +55,11 @@ export default function PoolDetailPage() {
   const blacklistSet = useMemo(() => new Set(blacklist), [blacklist]);
   const isMobile = useIsMobile();
   const zoom = useUi((state) => state.zoom);
+  // Pool posts load in chunks of the user's e621 page size (max 320).
+  const e621PerPage = useUi((state) => state.e621PerPage);
+  const chunk = Math.min(320, Math.max(24, e621PerPage || CHUNK_FALLBACK));
 
-  const [visibleCount, setVisibleCount] = useState(CHUNK);
+  const [visibleCount, setVisibleCount] = useState(chunk);
   const [showBlacklisted, setShowBlacklisted] = useState(false);
 
   const poolQuery = useQuery({
@@ -84,6 +88,7 @@ export default function PoolDetailPage() {
       fetchPostsChunked(
         effectiveCredentials(credentials),
         postIds,
+        chunk,
         signal,
       ),
     enabled: postIds.length > 0,
@@ -319,7 +324,7 @@ export default function PoolDetailPage() {
             <Button
               variant="secondary"
               disabled={postsQuery.isFetching}
-              onClick={() => setVisibleCount((count) => count + CHUNK)}
+              onClick={() => setVisibleCount((count) => count + chunk)}
             >
               {postsQuery.isFetching
                 ? "Loading…"