test: Playwright E2E with real backend + fake TTS engine

e2e/ drives the web build (vite preview) against a per-worker uvicorn:
each Playwright worker boots its own backend on its own port with a
temp cwd, so SQLite and audio files are fully isolated and parallel-
safe. The page fixture seeds the persisted voicebox-server store with
the worker's URL before any script runs.

VOICEBOX_FAKE_TTS=1 short-circuits get_tts_backend_for_engine to a
backend that synthesizes a sine tone sized to the text — the real task
queue, SSE progress, history rows, and audio serving all run, only
inference is fake. backend/requirements-ci.txt is the slim dependency
set validated to boot the app on a CPU-only runner.

Specs: startup, settings layout, seeded profile in /voices, and
generate-to-completed-audio through the full pipeline.

e2e/playwright.config.ts

@@ -0,0 +1,36 @@
+import { defineConfig, devices } from '@playwright/test';
+
+/**
+ * E2E suite driving the web build (same app as Tauri, browser platform)
+ * against a real CPU backend with the fake TTS engine. Each worker gets
+ * its own uvicorn on its own port with its own data dir — see fixtures.ts.
+ *
+ * PW_DEV=1 targets `bun run dev:web` (port 5173) instead of the preview
+ * build for faster local iteration.
+ */
+const DEV = !!process.env.PW_DEV;
+const PORT = DEV ? 5173 : 4173;
+
+export default defineConfig({
+  testDir: './specs',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 1 : 0,
+  reporter: process.env.CI ? [['html', { open: 'never' }], ['github']] : 'list',
+  use: {
+    baseURL: `http://localhost:${PORT}`,
+    trace: 'on-first-retry',
+    video: 'retain-on-failure',
+    screenshot: 'only-on-failure',
+  },
+  projects: [{ name: 'chromium', use: { ...devices['Desktop Chrome'] } }],
+  webServer: {
+    command: DEV
+      ? 'bun run dev:web'
+      : 'bun run build:web && cd web && bunx vite preview --port 4173 --strictPort',
+    cwd: '..',
+    port: PORT,
+    reuseExistingServer: !process.env.CI,
+    timeout: 120_000,
+  },
+});