Add Playwright e2e test infrastructure

Set up browser-based end-to-end testing using pytest-playwright:
- Add playwright-driver and pytest-playwright to nix flake
- Configure PLAYWRIGHT_BROWSERS_PATH for NixOS compatibility
- Create ServerHarness to manage live server for tests
- Add smoke tests for health endpoint and page loading
- Exclude e2e tests from pre-commit hook (require special setup)

Run e2e tests with: pytest tests/e2e/ -v -n 0

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/e2e/test_smoke.py

@@ -0,0 +1,29 @@
+# ABOUTME: Basic smoke tests to verify the e2e test setup works.
+# ABOUTME: Tests server startup, health endpoint, and page loading.
+
+import pytest
+import requests
+from playwright.sync_api import Page, expect
+
+pytestmark = pytest.mark.e2e
+
+
+def test_healthz_endpoint(live_server):
+    """Verify health endpoint returns OK."""
+    response = requests.get(f"{live_server.url}/healthz")
+    assert response.status_code == 200
+    assert response.text == "OK"
+
+
+def test_home_page_loads(page: Page, live_server):
+    """Verify the home page loads successfully."""
+    page.goto(live_server.url)
+    # Should see the page body
+    expect(page.locator("body")).to_be_visible()
+
+
+def test_animals_page_accessible(page: Page, live_server):
+    """Verify animals list page is accessible."""
+    page.goto(f"{live_server.url}/animals")
+    # Should see some content (exact content depends on seed data)
+    expect(page.locator("body")).to_be_visible()