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)

backend/apps/accounts/tests/test_preferences.py

@@ -0,0 +1,49 @@
+"""Per-user browse preferences: validation and merge semantics."""
+
+import json
+
+from django.contrib.auth import get_user_model
+from django.test import Client, TestCase
+
+from rest_framework.authtoken.models import Token
+
+User = get_user_model()
+
+
+class PreferenceTests(TestCase):
+    def setUp(self):
+        self.user = User.objects.create_user(
+            username="prefs-user", password="prefs-pass-123456"
+        )
+        self.client = Client()
+        self.client.defaults["HTTP_AUTHORIZATION"] = (
+            f"Token {Token.objects.create(user=self.user).key}"
+        )
+
+    def post(self, payload):
+        return self.client.post(
+            "/api/auth/preferences/",
+            data=json.dumps(payload),
+            content_type="application/json",
+        )
+
+    def test_e621_per_page_round_trips_within_range(self):
+        response = self.post({"e621_per_page": 320})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json()["e621_per_page"], 320)
+        self.assertEqual(
+            self.client.get("/api/auth/me/").json()["preferences"]["e621_per_page"],
+            320,
+        )
+
+    def test_e621_per_page_rejects_out_of_range(self):
+        for value in (0, 8, 321, 1000):
+            self.assertEqual(self.post({"e621_per_page": value}).status_code, 400, value)
+
+    def test_merge_keeps_other_keys(self):
+        self.post({"ratings": ["s"], "e621_per_page": 100})
+        response = self.post({"per_page": 24})
+        data = response.json()
+        self.assertEqual(data["ratings"], ["s"])
+        self.assertEqual(data["e621_per_page"], 100)
+        self.assertEqual(data["per_page"], 24)