feat(admin): Phase 0 平台超管基础 — is_platform_admin + IsPlatformAdmin + create_platform_admin(admin/admin123) + AdminAuditLog + Admin 后台外壳与路由 gating

后端:User.is_platform_admin + migration;权限类 IsPlatformAdmin;管理命令建 admin/admin123(幂等);
AdminAuditLog 模型 + log_admin_action() helper;me/login 对无团队超管优雅返回 team=null;UserSerializer 暴露标志。
前端:routes/admin 后台外壳(分组侧栏 + 概览 + 占位)、/admin 路由解析与 gating(超管直落、非超管纠回)、
侧栏平台入口、admin-page.css(仅 token)、IconKitSvg 补图标。
测试:accounts 11/11 单测过;无头 e2e _admin-p0.mjs 全断言过 + 0 console error;tsc+build 绿。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/accounts/migrations/0004_user_is_platform_admin_adminauditlog.py

@@ -0,0 +1,42 @@
+# Generated by Django 5.1.15 on 2026-06-19 12:35
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0003_invitation'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='is_platform_admin',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.CreateModel(
+            name='AdminAuditLog',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('operator_name', models.CharField(blank=True, max_length=150)),
+                ('action', models.CharField(max_length=48)),
+                ('target_type', models.CharField(blank=True, max_length=32)),
+                ('target_id', models.CharField(blank=True, max_length=64)),
+                ('target_name', models.CharField(blank=True, max_length=200)),
+                ('before', models.JSONField(blank=True, null=True)),
+                ('after', models.JSONField(blank=True, null=True)),
+                ('ip_address', models.GenericIPAddressField(blank=True, null=True)),
+                ('operator', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='admin_audit_logs', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['-created_at'],
+                'indexes': [models.Index(fields=['action'], name='accounts_ad_action_46dc9d_idx'), models.Index(fields=['-created_at'], name='accounts_ad_created_1ae9d4_idx')],
+            },
+        ),
+    ]