v0.0.36 | Upbit API 자격증명 관리 기능 추가

- CustomUser 모델에 Upbit access/secret key 암호화 필드 추가
- UpbitCredentialsView: 자격증명 저장/조회(마스킹)/삭제 API
- UpbitSecretKeyView: 복호화된 키 조회 API (내부 서비스 호출용)
- 마이그레이션 파일 추가

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

users/migrations/0016_customuser_encrypted_upbit_access_key_and_more.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.2.11 on 2026-02-16 01:34
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('users', '0015_customuser_locked_until_customuser_login_failures_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='customuser',
+            name='encrypted_upbit_access_key',
+            field=models.BinaryField(blank=True, null=True, verbose_name='Upbit Access Key (암호화)'),
+        ),
+        migrations.AddField(
+            model_name='customuser',
+            name='encrypted_upbit_secret_key',
+            field=models.BinaryField(blank=True, null=True, verbose_name='Upbit Secret Key (암호화)'),
+        ),
+    ]