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.

backend/config/settings.py

@@ -170,6 +170,10 @@ GUEST_BLACKLIST_TTL = int(os.getenv("GUEST_BLACKLIST_TTL", "3600"))
 # Similarity threshold for flagging staged uploads that match library items.
 VISUAL_MATCH_THRESHOLD = float(os.getenv("VISUAL_MATCH_THRESHOLD", "0.9"))
 
+# Ephemeral similarity-check uploads are deleted after this many minutes
+# (and always on startup).
+SIMILARITY_TTL_MINUTES = int(os.getenv("SIMILARITY_TTL_MINUTES", "30"))
+
 # Redis cache (run via docker compose at the repo root), shared by web
 # workers and management commands (e.g. the mirrored guest blacklist).
 CACHES = {