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.

frontend/src/App.tsx

@@ -16,6 +16,7 @@ import RegisterPage from "@/features/auth/RegisterPage";
 import DetailPage from "@/features/detail/DetailPage";
 import DeletePage from "@/features/library/DeletePage";
 import DuplicatesPage from "@/features/library/DuplicatesPage";
+import FollowedPage from "@/features/follows/FollowedPage";
 import LibraryPage from "@/features/library/LibraryPage";
 import Md5Redirect from "@/features/library/Md5Redirect";
 import OnlinePage from "@/features/online/OnlinePage";
@@ -70,6 +71,14 @@ export default function App() {
         <Route path="/view/:md5" element={<Md5Redirect />} />
         <Route path="/online" element={<OnlinePage />} />
         <Route path="/online/view/:postId" element={<PostRedirect />} />
+        <Route
+          path="/followed"
+          element={
+            <RequireAuth>
+              <FollowedPage />
+            </RequireAuth>
+          }
+        />
         <Route
           path="/duplicates"
           element={