Add GDPR-compliant cookie consent banner using Silktide

Implements Silktide Consent Manager via jsDelivr CDN for GDPR compliance.
The banner informs users the site only uses essential cookies and allows
them to manage preferences.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app.py

@@ -67,7 +67,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
         response.headers["Content-Security-Policy"] = (
             "default-src 'self'; "
             "script-src 'self' 'unsafe-inline' https://cdn.jsdelivr.net; "
-            "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; "
+            "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com https://cdn.jsdelivr.net; "
             "font-src 'self' https://fonts.gstatic.com; "
             "img-src 'self' data:; "
             "connect-src 'self' https://cdn.jsdelivr.net; "