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.

ROADMAP.md

@@ -44,9 +44,17 @@ they land.
         using the signed raw file)
   - [x] Show candidate posts to link (thumbnail, rating, score, favs, tags;
         select then confirm — same flow as uploads, exact MD5 marked)
-- [ ] **Follows**
-  - [ ] Follow tags/pools, Followed screens with unseen badges
-  - [ ] "Followed" nav entry
+- [x] **Follows**
+  - [x] Follow tags/pools from the `/followed` page (validated against e621,
+        first feed fetched immediately)
+  - [x] Followed screens: cover cards (latest image), unseen badges and
+        Mark seen per follow
+  - [x] Merged newest-first feed with per-follow filter and unseen-only toggle
+  - [x] Blacklisted-tag cloud built in a daemon thread, then polled
+        (10 min TTL; the user's e621 blacklist, guest default as fallback)
+  - [x] "Followed" nav entry
+  - [x] Periodic sync commands: `sync_followed_tags` and
+        `sync_followed_pools` (one e621 search per unique tag/pool)
 - [ ] `F` shortcut to favorite a post, tag finder in the Ctrl+K palette
 
 ## 3. Uploads & ingestion pipeline
@@ -99,6 +107,8 @@ Files now stage first and are resolved before entering the library.
 ## 6. Infrastructure
 
 - [ ] Guest blacklist refresh on a timer (`refresh_guest_blacklist` via cron/systemd)
+- [ ] Follow sync on a timer (`sync_followed_tags` + `sync_followed_pools` via
+      cron/systemd, e.g. every 30 minutes)
 - [ ] Production setup: build the SPA, serve via Nginx (static + `/media` +
       `/library`), systemd unit for Waitress
 - [ ] Automated tests (backend API + frontend components)