Phase 3: J-IDs, ownership, roles, guest safety, adaptive detail, download

Backend:
- User.role (user/uploader/staff) with can_upload; uploads and downloads
  gated to uploader+; owners and staff can edit their items
- MediaItem.uploaded_by plus J-<id> identity (serializer, admin,
  scan_files --user, first superuser as default owner)
- API resolves J-<id>, bare numeric ids and MD5s; neighbors and lookup
  return j_ids
- Guest safety: mirror e621's anonymous default blacklist into Redis
  (parses comments, negations and wildcards), flag hidden_from_guests
  and filter lists, details and lookups for anonymous users
- POST /api/online/downloads/ writes an e621 file into the watched
  folder and indexes it for the uploader
- MariaDB + Redis via docker compose (host ports 3307/6380), PyMySQL
  driver shim, Redis cache replacing the file cache; SQLite data
  dumped and loaded into MariaDB

Frontend:
- Single /detail/:itemId route with an adaptive shell: J-<id> renders
  the library item, bare numbers render the e621 post
- Legacy /view/<md5> and /online/view/<id> redirect to canonical URLs
- Cards expose J-IDs; library custom-data editor is read-only for
  non-owners
- Role gating: Upload hidden/blocked for regular users, account shows
  the role, guest hint on the library

backend/apps/library/urls.py

@@ -1,7 +1,7 @@
 from django.urls import include, path
 from rest_framework.routers import DefaultRouter
 
-from .views import MediaItemViewSet, UploadView
+from .views import DownloadToLibraryView, MediaItemViewSet, UploadView
 
 router = DefaultRouter()
 router.register("files", MediaItemViewSet, basename="file")
@@ -9,4 +9,9 @@ router.register("files", MediaItemViewSet, basename="file")
 urlpatterns = [
     path("", include(router.urls)),
     path("uploads/", UploadView.as_view(), name="upload"),
+    path(
+        "online/downloads/",
+        DownloadToLibraryView.as_view(),
+        name="download_to_library",
+    ),
 ]