fix: CSP — ajout cloudflareinsights.com + legacy.js deja corrige

- CSP script-src: ajout https://static.cloudflareinsights.com
- legacy.js ligne 98 a deja le null-check if(filterBtn)
- L'erreur production vient d'un ancien deploy, sera resolu au prochain rebuild

backend/main.py

@@ -545,7 +545,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
         response.headers["Referrer-Policy"] = "strict-origin-when-cross-origin"
         response.headers["Content-Security-Policy"] = (
             "default-src 'self'; "
-            "script-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com https://unpkg.com https://esm.sh https://cdn.jsdelivr.net; "
+            "script-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com https://unpkg.com https://esm.sh https://cdn.jsdelivr.net https://static.cloudflareinsights.com; "
             "style-src 'self' 'unsafe-inline' https://cdnjs.cloudflare.com https://fonts.googleapis.com https://cdn.jsdelivr.net; "
             "img-src 'self' data: blob:; "
             "connect-src 'self' https://esm.sh https://unpkg.com https://cdnjs.cloudflare.com https://fonts.googleapis.com https://fonts.gstatic.com https://cdn.jsdelivr.net; "