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/models.py

@@ -0,0 +1,114 @@
+from django.conf import settings
+from django.db import models
+
+
+class FollowedTag(models.Model):
+    """A tag one user subscribes to; feeds are filled by the sync command."""
+
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="followed_tags",
+    )
+    tag = models.CharField(max_length=200)
+    cover_data = models.JSONField(null=True, blank=True)
+    last_synced_at = models.DateTimeField(null=True, blank=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        ordering = ["tag"]
+        constraints = [
+            models.UniqueConstraint(
+                fields=["user", "tag"], name="uniq_followed_tag_per_user"
+            ),
+        ]
+
+    def __str__(self):
+        return f"{self.user} follows tag:{self.tag}"
+
+
+class FollowedPool(models.Model):
+    """A pool one user subscribes to; feeds are filled by the sync command."""
+
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="followed_pools",
+    )
+    pool_id = models.IntegerField()
+    name = models.CharField(max_length=200, blank=True, default="")
+    post_count = models.IntegerField(default=0)
+    cover_data = models.JSONField(null=True, blank=True)
+    last_synced_at = models.DateTimeField(null=True, blank=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        ordering = ["pool_id"]
+        constraints = [
+            models.UniqueConstraint(
+                fields=["user", "pool_id"], name="uniq_followed_pool_per_user"
+            ),
+        ]
+
+    def __str__(self):
+        return f"{self.user} follows pool:{self.pool_id}"
+
+
+class FollowedPost(models.Model):
+    """One post seen in a followed feed (stored once per follow source)."""
+
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="followed_posts",
+    )
+    tag = models.ForeignKey(
+        FollowedTag,
+        null=True,
+        blank=True,
+        on_delete=models.CASCADE,
+        related_name="posts",
+    )
+    pool = models.ForeignKey(
+        FollowedPool,
+        null=True,
+        blank=True,
+        on_delete=models.CASCADE,
+        related_name="posts",
+    )
+    post_id = models.IntegerField(db_index=True)
+    data = models.JSONField(default=dict)
+    seen = models.BooleanField(default=False, db_index=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        ordering = ["-post_id"]
+        indexes = [models.Index(fields=["user", "post_id"])]
+        constraints = [
+            models.UniqueConstraint(
+                fields=["tag", "post_id"], name="uniq_followed_tag_post"
+            ),
+            models.UniqueConstraint(
+                fields=["pool", "post_id"], name="uniq_followed_pool_post"
+            ),
+        ]
+
+    def __str__(self):
+        return f"#{self.post_id} for {self.user}"
+
+
+class FollowCloud(models.Model):
+    """Cached blacklisted-tag cloud for the Followed page, built in a thread."""
+
+    user = models.OneToOneField(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="follow_cloud",
+    )
+    data = models.JSONField(null=True, blank=True)
+    computed_at = models.DateTimeField(null=True, blank=True)
+    building = models.BooleanField(default=False)
+    building_since = models.DateTimeField(null=True, blank=True)
+
+    def __str__(self):
+        return f"Follow cloud for {self.user}"