Add an ephemeral similarity check page

- /similar (nav: Similar): drop a file to get the exact MD5 match, the
  perceptual matches against the library, and e621 IQDB candidates
  (auto-run for images when credentials are configured). Read-only —
  nothing enters the library.
- SimilarityCheck model + /api/similarity/ (create/list/retrieve/delete)
  with signed preview URLs and an expires_at timestamp.
- Temp files are wiped on startup (AppConfig.ready, file-only so no
  database access during initialization), lazily past
  SIMILARITY_TTL_MINUTES (default 30, env-overridable), on delete, and
  by manage.py cleanup_similarity.
- uploadFile() takes a target path; .env.example documents the TTL.

frontend/src/App.tsx

@@ -22,6 +22,7 @@ import Md5Redirect from "@/features/library/Md5Redirect";
 import OnlinePage from "@/features/online/OnlinePage";
 import PoolDetailPage from "@/features/pools/PoolDetailPage";
 import PoolsPage from "@/features/pools/PoolsPage";
+import SimilarPage from "@/features/similar/SimilarPage";
 import UploadPage from "@/features/upload/UploadPage";
 import UsersPage from "@/features/users/UsersPage";
 import { isAgeVerified, markAgeVerified } from "@/lib/age";
@@ -107,6 +108,14 @@ export default function App() {
             </RequireAuth>
           }
         />
+        <Route
+          path="/similar"
+          element={
+            <RequireAuth>
+              <SimilarPage />
+            </RequireAuth>
+          }
+        />
         <Route
           path="/upload"
           element={