Upload pipeline: staging, MD5 auto-match, IQDB, three-column board

Backend:
- TempUpload model: staged files (pending / visual_match / completed /
  error) with resolution, e621 payload, custom metadata and IQDB data
- Files land in a temp folder and only move into the watched library
  folder once resolved; duplicates resolve immediately without a copy
- Endpoints: stage (multipart), list, retrieve, temp file, IQDB save,
  resolve (link to post or custom metadata), discard/dismiss
- cleanup_temp_uploads command for old staged files
- Replaces the old direct-to-library upload endpoint

Frontend:
- Upload page is now a three-column board (Pending & Unmatched /
  Visual Similarity Detected / Auto-uploaded & Indexed)
- After upload: MD5s are batch-checked against e621 and matches
  auto-complete with full post metadata; remaining files run through
  IQDB and move to the similarity column when candidates exist
- Metadata modal with IQDB candidates, post-ID linking and custom
  tags/rating/notes; discard and dismiss actions
- e621 client gains fetchPostsByMd5 and iqdbSearch helpers

Roadmap updated with the completed upload items.

backend/apps/library/management/commands/cleanup_temp_uploads.py

@@ -0,0 +1,35 @@
+from datetime import timedelta
+
+from django.core.management.base import BaseCommand
+from django.utils import timezone
+
+from apps.library.models import TempUpload
+
+
+class Command(BaseCommand):
+    help = "Delete staged uploads older than N hours (default 24)."
+
+    def add_arguments(self, parser):
+        parser.add_argument("--hours", type=int, default=24)
+        parser.add_argument(
+            "--include-completed",
+            action="store_true",
+            help="Also remove completed staging records.",
+        )
+
+    def handle(self, *args, **options):
+        cutoff = timezone.now() - timedelta(hours=options["hours"])
+        queryset = TempUpload.objects.filter(created_at__lt=cutoff)
+        if not options["include_completed"]:
+            queryset = queryset.exclude(status=TempUpload.STATUS_COMPLETED)
+
+        removed = 0
+        for temp in queryset:
+            if temp.file:
+                temp.file.delete(save=False)
+            temp.delete()
+            removed += 1
+
+        self.stdout.write(
+            self.style.SUCCESS(f"Removed {removed} staged upload(s).")
+        )