integration: add Playwright, axe and Lighthouse CI gates

Lock SEO, accessibility and crawlability with headless browser checks so regressions fail before a merge instead of after publication.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -41,3 +41,9 @@ __screenshots__/
 # System files
 .DS_Store
 Thumbs.db
+
+# Browser and audit artifacts
+/test-results
+/playwright-report
+/blob-report
+/.lighthouseci