Add Playwright E2E harness for the RDM audit workspace

Why: U4 acceptance requires real-browser runs of the trace/audit
workspace; the repo had only a manual .playwright.ts stub gated on
PLAYWRIGHT_BASE_URL with no config, deps, or service wiring.

Evidence: config/deps/spec staged; full run driven by Dev-side
scripts/e2e.sh (seed-only smoke in progress in this task branch).

Impact: dev-only additions (@playwright/test devDep, preview proxy
segment, e2e/rdm-audit.playwright.ts); no production bundle changes.

Refs: Dev task feature/rdm-e2e-acceptance; U4 in
modules/documents/docs/proposals/2026-10-11-rdm-two-trees-trace-audit-integration.md

playwright.config.ts

@@ -0,0 +1,20 @@
+// U4 运行验收（Playwright）：真实浏览器走通追溯审计工作区四入口。
+// 由 Dev 仓 scripts/e2e.sh 驱动（起 RDM 二进制 + preview 静态前端 + 种子数据）；
+// PLAYWRIGHT_BASE_URL 未设置时整组跳过（CI 单测档不拉浏览器）。
+import { defineConfig } from '@playwright/test'
+
+const base = process.env.PLAYWRIGHT_BASE_URL || ''
+
+export default defineConfig({
+  testDir: 'e2e',
+  testMatch: /.*(spec|playwright)\.ts$/,
+  timeout: 30_000,
+  retries: 0,
+  workers: 1,
+  reporter: [['list']],
+  use: {
+    baseURL: base,
+    locale: 'zh-CN',
+    trace: 'retain-on-failure',
+  },
+})