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/serializers.py

@@ -5,9 +5,19 @@ from .models import User
 
 
 class UserSerializer(serializers.ModelSerializer):
+    e621_configured = serializers.BooleanField(read_only=True)
+
     class Meta:
         model = User
-        fields = ["id", "username", "email", "is_staff", "date_joined"]
+        fields = [
+            "id",
+            "username",
+            "email",
+            "is_staff",
+            "date_joined",
+            "e621_username",
+            "e621_configured",
+        ]
         read_only_fields = fields
 
 
@@ -20,3 +30,13 @@ class RegisterSerializer(serializers.ModelSerializer):
 
     def create(self, validated_data):
         return User.objects.create_user(**validated_data)
+
+
+class E621CredentialsSerializer(serializers.Serializer):
+    username = serializers.CharField(max_length=100, allow_blank=True)
+    api_key = serializers.CharField(
+        max_length=100, allow_blank=True, trim_whitespace=True
+    )
+    base_url = serializers.URLField(
+        max_length=200, required=False, allow_blank=True
+    )