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/management/commands/cleanup_similarity.py

@@ -0,0 +1,46 @@
+from datetime import timedelta
+
+from django.conf import settings
+from django.core.management.base import BaseCommand
+from django.utils import timezone
+
+from apps.library.similarity import purge_similarity_checks
+
+
+class Command(BaseCommand):
+    help = (
+        "Delete ephemeral similarity-check files. By default removes checks "
+        "older than SIMILARITY_TTL_MINUTES; use --all to wipe every one."
+    )
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            "--minutes",
+            type=int,
+            default=None,
+            help="Delete checks older than N minutes "
+            "(default: the configured TTL, currently "
+            f"{getattr(settings, 'SIMILARITY_TTL_MINUTES', 30)}).",
+        )
+        parser.add_argument(
+            "--all",
+            action="store_true",
+            help="Delete every similarity check regardless of age.",
+        )
+
+    def handle(self, *args, **options):
+        if options["all"]:
+            deleted = purge_similarity_checks()
+            scope = "all"
+        else:
+            minutes = (
+                options["minutes"]
+                if options["minutes"] is not None
+                else getattr(settings, "SIMILARITY_TTL_MINUTES", 30)
+            )
+            cutoff = timezone.now() - timedelta(minutes=minutes)
+            deleted = purge_similarity_checks(cutoff)
+            scope = f"older than {minutes} minute(s)"
+        self.stdout.write(
+            self.style.SUCCESS(f"Deleted {deleted} similarity check(s) ({scope}).")
+        )