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/library/e621.py

@@ -41,13 +41,17 @@ def _wait_for_slot():
         _last_request_at = time.monotonic()
 
 
-def get(user, path, params=None, timeout=30):
+def get(user, path, params=None, timeout=30, require_auth=True):
     """GET an e621 API path using the user's credentials.
 
+    Reads that e621 serves anonymously (searches, pools, tags) can pass
+    require_auth=False; matching endpoints keep requiring credentials.
+
     Raises E621NotFound for 404s and E621Error for everything else that isn't
     a 2xx, so callers never see requests exceptions.
     """
-    if not credentials_configured(user):
+    configured = credentials_configured(user)
+    if require_auth and not configured:
         raise E621Error("Configure your e621 credentials in Account first.")
     base = (getattr(user, "e621_base_url", "") or "https://e621.net").rstrip("/")
     _wait_for_slot()
@@ -55,7 +59,7 @@ def get(user, path, params=None, timeout=30):
         response = requests.get(
             f"{base}{path}",
             params=params,
-            auth=(user.e621_username, user.e621_api_key),
+            auth=(user.e621_username, user.e621_api_key) if configured else None,
             headers={"User-Agent": settings.USER_AGENT},
             timeout=timeout,
         )

backend/apps/library/services.py

@@ -323,6 +323,9 @@ def trim_e621_post(post):
             "height": file_data.get("height"),
             "url": file_data.get("url"),
         },
+        "preview": {
+            "url": (post.get("preview") or {}).get("url"),
+        },
         "uploader_name": post.get("uploader_name"),
     }