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/accounts/serializers.py

@@ -134,5 +134,7 @@ class PreferencesSerializer(serializers.Serializer):
     ordering = serializers.CharField(max_length=30, required=False)
     per_page = serializers.IntegerField(min_value=12, max_value=200, required=False)
     zoom = serializers.IntegerField(min_value=120, max_value=400, required=False)
+    # e621 renders at most 320 posts per request.
+    e621_per_page = serializers.IntegerField(min_value=12, max_value=320, required=False)
     # Open Online with the order:hot metatag when no search is given.
     online_hot_default = serializers.BooleanField(required=False)