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>

lighthouserc.json

@@ -0,0 +1,31 @@
+{
+  "ci": {
+    "collect": {
+      "startServerCommand": "npm run serve:ssr",
+      "startServerReadyPattern": "Node Express server listening",
+      "url": [
+        "http://127.0.0.1:4000/",
+        "http://127.0.0.1:4000/en",
+        "http://127.0.0.1:4000/leistungen/ai-integration",
+        "http://127.0.0.1:4000/en/projects"
+      ],
+      "numberOfRuns": 3,
+      "settings": {
+        "preset": "desktop",
+        "chromeFlags": "--no-sandbox --headless=new"
+      }
+    },
+    "assert": {
+      "assertions": {
+        "categories:performance": ["error", { "minScore": 0.85 }],
+        "categories:accessibility": ["error", { "minScore": 1 }],
+        "categories:best-practices": ["error", { "minScore": 1 }],
+        "categories:seo": ["error", { "minScore": 1 }]
+      }
+    },
+    "upload": {
+      "target": "filesystem",
+      "outputDir": ".lighthouseci"
+    }
+  }
+}