Add Playwright E2E test suite (25 tests across 4 specs)

- playwright.config.ts: headless CI setup with JSON/HTML reporters
- e2e/global-setup.ts: app reachability check before tests
- e2e/auth.spec.ts: 6 auth tests (sign-in, session, protected routes)
- e2e/compare.spec.ts: 8 API tests (validation, SSE flow, DB persistence)
- e2e/comparisons.spec.ts: 6 CRUD tests (list, detail, 404, view counts)
- e2e/user.spec.ts: 5 user tests (stats, pagination, session binding)
- e2e/helpers.ts: shared SSE parsing, auth, URL resolution utilities
- e2e/README.md: setup and run instructions
- AGENTS.md: updated with LLM provider notes and testing docs
- .gitignore: added playwright report/result directories
- package.json: added @playwright/test and test:e2e scripts

e2e/global-teardown.ts

@@ -0,0 +1,9 @@
+import type { FullConfig } from "@playwright/test";
+
+/**
+ * Global teardown — runs once after all tests finish.
+ * No-op placeholder for cleanup hooks.
+ */
+export default async function globalTeardown(_config: FullConfig) {
+  // Add cleanup logic here if needed (e.g., stop test containers)
+}