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/apps/library/urls.py

@@ -10,6 +10,7 @@ from .tools import (
     VisualGroupsView,
     VisualMatchesView,
 )
+from .similarity import SimilarityCheckViewSet
 from .uploads import TempUploadViewSet
 from .views import (
     ClientDownloadView,
@@ -23,6 +24,7 @@ router.register("files", MediaItemViewSet, basename="file")
 router.register("uploads", TempUploadViewSet, basename="upload")
 router.register("online/downloads", DownloadTaskViewSet, basename="download")
 router.register("matches", MatchTaskViewSet, basename="match")
+router.register("similarity", SimilarityCheckViewSet, basename="similarity")
 
 urlpatterns = [
     path("", include(router.urls)),