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/serializers.py

@@ -2,7 +2,7 @@ import os
 
 from rest_framework import serializers
 
-from .models import MediaItem, MediaLocation
+from .models import MediaItem, MediaLocation, TempUpload
 from .services import VIDEO_EXTENSIONS
 
 
@@ -92,3 +92,37 @@ class MediaItemSerializer(serializers.ModelSerializer):
 
     def get_kind(self, obj):
         return "video" if "." + self.get_extension(obj) in VIDEO_EXTENSIONS else "image"
+
+
+class TempUploadSerializer(serializers.ModelSerializer):
+    temp_id = serializers.UUIDField(source="id", read_only=True)
+    library_j_id = serializers.SerializerMethodField()
+    file_url = serializers.SerializerMethodField()
+
+    class Meta:
+        model = TempUpload
+        fields = [
+            "temp_id",
+            "original_filename",
+            "md5",
+            "size",
+            "status",
+            "resolution",
+            "e621_post_id",
+            "e621_data",
+            "custom_rating",
+            "custom_tags",
+            "custom_notes",
+            "iqdb_data",
+            "library_j_id",
+            "file_url",
+            "created_at",
+            "updated_at",
+        ]
+        read_only_fields = fields
+
+    def get_library_j_id(self, obj):
+        return f"J-{obj.library_item_id}" if obj.library_item_id else None
+
+    def get_file_url(self, obj):
+        return f"/api/uploads/{obj.id}/file/" if obj.file else None