Add Google Analytics 4 with cookie consent integration

- Add GA4 measurement ID to config (default: G-J0PCVT14NY)
- Add /api/config endpoint to expose GA ID to frontend
- Update cookie consent with Analytics category (opt-in)
- Load GA4 only after user consents to analytics cookies
- Update CSP to allow Google Analytics domains

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

backend/app.py

@@ -65,11 +65,11 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
         # Content Security Policy
         response.headers["Content-Security-Policy"] = (
             "default-src 'self'; "
-            "script-src 'self' 'unsafe-inline' https://cdn.jsdelivr.net https://unpkg.com; "
+            "script-src 'self' 'unsafe-inline' https://cdn.jsdelivr.net https://unpkg.com https://www.googletagmanager.com; "
             "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com https://cdn.jsdelivr.net https://unpkg.com; "
             "font-src 'self' https://fonts.gstatic.com; "
-            "img-src 'self' data: https://*.tile.openstreetmap.org https://unpkg.com; "
-            "connect-src 'self' https://cdn.jsdelivr.net https://*.tile.openstreetmap.org https://unpkg.com; "
+            "img-src 'self' data: https://*.tile.openstreetmap.org https://unpkg.com https://www.google-analytics.com; "
+            "connect-src 'self' https://cdn.jsdelivr.net https://*.tile.openstreetmap.org https://unpkg.com https://www.google-analytics.com https://analytics.google.com https://*.google-analytics.com; "
             "frame-ancestors 'none'; "
             "base-uri 'self'; "
             "form-action 'self';"
@@ -199,6 +199,14 @@ async def serve_rankings():
     return FileResponse(settings.frontend_dir / "index.html")
 
 
+@app.get("/api/config")
+async def get_config():
+    """Return public configuration for the frontend."""
+    return {
+        "ga_measurement_id": settings.ga_measurement_id
+    }
+
+
 @app.get("/api/schools")
 @limiter.limit(f"{settings.rate_limit_per_minute}/minute")
 async def get_schools(