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).

backend/apps/follows/views.py

@@ -36,10 +36,16 @@ class FollowedTagViewSet(
     mixins.DestroyModelMixin,
     viewsets.GenericViewSet,
 ):
-    """Tags the current user follows."""
+    """Tags the current user follows.
+
+    Deliberately unpaginated: the SPA treats this as the complete set (follow
+    toggles read their state from it and the Followed page lists every card),
+    so a 48-item page silently broke following past 48 entries.
+    """
 
     serializer_class = FollowedTagSerializer
     permission_classes = [IsAuthenticated]
+    pagination_class = None
     http_method_names = ["get", "post", "delete", "head", "options"]
 
     def get_queryset(self):
@@ -112,10 +118,11 @@ class FollowedPoolViewSet(
     mixins.DestroyModelMixin,
     viewsets.GenericViewSet,
 ):
-    """Pools the current user follows."""
+    """Pools the current user follows (unpaginated, like the tag list)."""
 
     serializer_class = FollowedPoolSerializer
     permission_classes = [IsAuthenticated]
+    pagination_class = None
     http_method_names = ["get", "post", "delete", "head", "options"]
 
     def get_queryset(self):