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/components/AppShell.tsx

@@ -62,6 +62,7 @@ export function AppShell() {
     { to: "/online", label: "Online" },
     { to: "/pools", label: "Pools" },
     ...(user ? [{ to: "/followed", label: "Followed" }] : []),
+    ...(user ? [{ to: "/similar", label: "Similar" }] : []),
     ...(user?.can_upload
       ? [
           { to: "/upload", label: "Upload" },