Add per-host connection limits for upstream fetching

- Add upstream_connections_per_host config option (default: 20)
- Implement per-host semaphores to limit concurrent connections
- Semaphore released when response body is closed
- Prevents overwhelming origin servers with parallel requests

example-config.yml

@@ -22,6 +22,9 @@ whitelist_hosts:
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
 allow_http: false
 
+# Maximum concurrent connections per upstream host (default: 20)
+upstream_connections_per_host: 20
+
 # Sentry error reporting (optional)
 sentry_dsn: ""