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/apps.py

@@ -1,5 +1,21 @@
+import logging
+
 from django.apps import AppConfig
 
+logger = logging.getLogger(__name__)
+
 
 class LibraryConfig(AppConfig):
     name = "apps.library"
+
+    def ready(self):
+        """Ephemeral similarity-check files never survive a restart."""
+        from . import similarity
+
+        try:
+            deleted = similarity.purge_similarity_files()
+        except OSError as exc:
+            logger.warning("Similarity file purge skipped: %s", exc)
+        else:
+            if deleted:
+                logger.info("Purged %s similarity file(s) on startup", deleted)