Online browser + post detail: SPA talks to e621 directly

Backend:
- POST /api/files/lookup/ reports which MD5s are already in the library

Frontend:
- e621 client extended with post/tag/favorite types and helpers: post
  search, post detail, batch posts by id, tag autocomplete, toggle
  favorite
- /online: tag search with autocomplete, post grid with rating colors
  and in-library badges, numbered pagination, page tag cloud, blacklist
  panel and filtered counts from /users/me.json, anonymous hint
- /online/view/🆔 media viewer (sample or original, video support),
  taxonomy-colored tags by category, specs sheet, favorite/unfavorite,
  description, sources, pools and parent/children thumbnails
- Shared CollapsibleSidebar extracted from the library page; Online
  added to the nav, command palette and sidebar toggle

backend/apps/library/views.py

@@ -5,7 +5,7 @@ from django.db.models import Min
 from rest_framework import mixins, status, viewsets
 from rest_framework.decorators import action
 from rest_framework.parsers import FormParser, MultiPartParser
-from rest_framework.permissions import IsAuthenticated
+from rest_framework.permissions import AllowAny, IsAuthenticated
 from rest_framework.response import Response
 from rest_framework.views import APIView
 
@@ -24,7 +24,7 @@ class MediaItemViewSet(
 ):
     serializer_class = MediaItemSerializer
     lookup_field = "md5"
-    http_method_names = ["get", "patch", "head", "options"]
+    http_method_names = ["get", "post", "patch", "head", "options"]
 
     def get_queryset(self):
         queryset = MediaItem.objects.prefetch_related("locations").annotate(
@@ -79,6 +79,19 @@ class MediaItemViewSet(
             return services.serve_file(request, thumbnail)
         return services.serve_file(request, path)
 
+    @action(detail=False, methods=["post"], permission_classes=[AllowAny])
+    def lookup(self, request):
+        """Return which of the given MD5s already exist in the library."""
+        md5s = request.data.get("md5s")
+        if not isinstance(md5s, list):
+            return Response(
+                {"detail": "md5s must be a list."},
+                status=status.HTTP_400_BAD_REQUEST,
+            )
+        wanted = {str(value).strip().lower()[:32] for value in md5s if value}
+        found = MediaItem.objects.filter(md5__in=wanted).values_list("md5", flat=True)
+        return Response({"found": sorted(found)})
+
     @action(detail=True, methods=["get"])
     def neighbors(self, request, md5=None):
         """Previous/next items in the ordered list, for keyboard navigation."""