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/models.py

@@ -1,5 +1,14 @@
 from django.contrib.auth.models import AbstractUser
+from django.db import models
 
 
 class User(AbstractUser):
-    """Project user. Extended later with avatars and e621 credentials."""
+    """Project user with optional e621 API credentials."""
+
+    e621_username = models.CharField(max_length=100, blank=True, default="")
+    e621_api_key = models.CharField(max_length=100, blank=True, default="")
+    e621_base_url = models.CharField(max_length=200, default="https://e621.net")
+
+    @property
+    def e621_configured(self):
+        return bool(self.e621_username and self.e621_api_key)