e621 credentials: user fields, API endpoint, Account screen

Backend:
- User model gains e621_username / e621_api_key / e621_base_url
- GET/PUT /api/auth/e621/ for the owner's credentials; /me exposes only
  the username and a configured flag, never the key

Frontend:
- e621 client core: Basic auth, _client param (browsers cannot set a
  User-Agent), serialized queue throttled to 1 request/second, readable
  error mapping
- Account screen (/account): username, API key with reveal toggle,
  base URL (e621 / e926 / custom), Save + Test connection
- Credentials are fetched from the backend and held in memory only,
  cleared on logout

backend/apps/accounts/urls.py

@@ -1,11 +1,12 @@
 from django.urls import path
 from rest_framework.authtoken.views import obtain_auth_token
 
-from .views import LogoutView, MeView, RegisterView
+from .views import E621CredentialsView, LogoutView, MeView, RegisterView
 
 urlpatterns = [
     path("register/", RegisterView.as_view(), name="register"),
     path("token/", obtain_auth_token, name="login"),
     path("logout/", LogoutView.as_view(), name="logout"),
     path("me/", MeView.as_view(), name="me"),
+    path("e621/", E621CredentialsView.as_view(), name="e621_credentials"),
 ]