feat(admin): 优化后台运维与任务管理

完善平台、基准模型、定价规则和实时负载的紧凑查询与滚动展示，并固定关键操作列。

新增管理员任务记录、批量计费结算和用户组限流、额度及模型权限维护能力，同时补充任务脱敏、查询索引与 OpenAPI 契约。

验证：pnpm openapi、Go 全量测试、pnpm lint、pnpm test、pnpm build、gofmt 与差异格式检查均通过。

apps/api/migrations/0087_admin_task_query_indexes.sql

@@ -0,0 +1,22 @@
+-- easyai:migration:no-transaction
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_gateway_tasks_admin_created
+  ON gateway_tasks(created_at DESC, id DESC);
+
+-- easyai:migration:statement
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_gateway_tasks_admin_status_created
+  ON gateway_tasks(status, created_at DESC, id DESC);
+
+-- easyai:migration:statement
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_gateway_tasks_admin_user_created
+  ON gateway_tasks(gateway_user_id, created_at DESC, id DESC)
+  WHERE gateway_user_id IS NOT NULL;
+
+-- easyai:migration:statement
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_gateway_tasks_admin_tenant_created
+  ON gateway_tasks(gateway_tenant_id, created_at DESC, id DESC)
+  WHERE gateway_tenant_id IS NOT NULL;
+
+-- easyai:migration:statement
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_gateway_task_attempts_admin_platform_task
+  ON gateway_task_attempts(platform_id, task_id)
+  WHERE platform_id IS NOT NULL;