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/urls.py

@@ -1,14 +1,15 @@
 from django.urls import include, path
 from rest_framework.routers import DefaultRouter
 
-from .views import DownloadToLibraryView, MediaItemViewSet, UploadView
+from .uploads import TempUploadViewSet
+from .views import DownloadToLibraryView, MediaItemViewSet
 
 router = DefaultRouter()
 router.register("files", MediaItemViewSet, basename="file")
+router.register("uploads", TempUploadViewSet, basename="upload")
 
 urlpatterns = [
     path("", include(router.urls)),
-    path("uploads/", UploadView.as_view(), name="upload"),
     path(
         "online/downloads/",
         DownloadToLibraryView.as_view(),