fix: CSP font-src blocks Excalidraw fonts from esm.sh

CSP directive 'font-src' only allowed self, data:, and fonts.gstatic.com.
Excalidraw loads its handwriting fonts (*.woff2) from esm.sh via
EXCALIDRAW_ASSET_PATH. All ~12 font files were blocked (blocked:csp)
→ Excalidraw fails to initialize → infinite 'Loading...' spinner.

Fix: added https://esm.sh to font-src directive.

backend/main.py

@@ -575,7 +575,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
             "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' blob: https://esm.sh https://unpkg.com https://cdnjs.cloudflare.com https://fonts.googleapis.com https://fonts.gstatic.com https://cdn.jsdelivr.net; "
-            "font-src 'self' data: https://fonts.gstatic.com; "
+            "font-src 'self' data: https://fonts.gstatic.com https://esm.sh; "
             "worker-src 'self' blob:; "
             "frame-src 'self' blob:;"
         )