Follows: followed tags/pools, feeds, unseen badges and blacklist cloud

Backend (new apps.follows):
- FollowedTag/FollowedPool/FollowedPost models; per-user follows with
  unseen tracking, plus FollowCloud for the cached blacklist cloud.
- Two periodic commands sharing one fetch path: sync_followed_tags and
  sync_followed_pools fetch each followed tag/pool's newest posts (one
  e621 search per unique follow), store unseen feed rows, refresh covers
  and pool metadata; both fall back to anonymous e621 access.
- API: /api/follows/tags|pools (follow, unfollow, mark seen), a merged
  feed with per-follow filtering, and /api/follows/cloud/ which rebuilds
  the blacklisted-tag cloud in a daemon thread when its 10 min cache is
  stale (polling returns building/ready).
- e621 client now supports anonymous reads; trimmed posts carry preview
  URLs for covers and feed tiles.

Frontend:
- /followed page: follow forms, cover cards with unseen badges and
  Mark seen, merged feed with filter/unseen toggle, and a blacklist
  cloud panel that polls while building. Followed nav entry added.

backend/apps/follows/serializers.py

@@ -0,0 +1,53 @@
+from rest_framework import serializers
+
+from .models import FollowedPool, FollowedTag
+
+
+class FollowedTagSerializer(serializers.ModelSerializer):
+    unseen_count = serializers.IntegerField(read_only=True)
+
+    class Meta:
+        model = FollowedTag
+        fields = [
+            "id",
+            "tag",
+            "unseen_count",
+            "cover_data",
+            "last_synced_at",
+            "created_at",
+        ]
+        read_only_fields = [
+            "id",
+            "tag",
+            "unseen_count",
+            "cover_data",
+            "last_synced_at",
+            "created_at",
+        ]
+
+
+class FollowedPoolSerializer(serializers.ModelSerializer):
+    unseen_count = serializers.IntegerField(read_only=True)
+
+    class Meta:
+        model = FollowedPool
+        fields = [
+            "id",
+            "pool_id",
+            "name",
+            "post_count",
+            "unseen_count",
+            "cover_data",
+            "last_synced_at",
+            "created_at",
+        ]
+        read_only_fields = [
+            "id",
+            "pool_id",
+            "name",
+            "post_count",
+            "unseen_count",
+            "cover_data",
+            "last_synced_at",
+            "created_at",
+        ]