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/migrations/0002_user_e621_api_key_user_e621_base_url_and_more.py

@@ -0,0 +1,28 @@
+# Generated by Django 6.1.1 on 2026-09-17 13:47
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='e621_api_key',
+            field=models.CharField(blank=True, default='', max_length=100),
+        ),
+        migrations.AddField(
+            model_name='user',
+            name='e621_base_url',
+            field=models.CharField(default='https://e621.net', max_length=200),
+        ),
+        migrations.AddField(
+            model_name='user',
+            name='e621_username',
+            field=models.CharField(blank=True, default='', max_length=100),
+        ),
+    ]