chore: changelog for 21st 2025 (#3000)

Author: NicholasKissel

site/src/posts/2025-09-21-weekly-updates/image.png

@@ -0,0 +1,11 @@
+export const author = "nicholas-kissel"
+export const published = "2025-09-26"
+export const category = "changelog"
+export const keywords = ["weekly-update", "engine", "admin-token", "live-chat", "custom-headers", "performance"]
+
+# Rivet Weekly Changelog 2025-38
+
+- Configurable admin token for the Engine
+- Live chat for feedback & bug requests in Engine & Inspector
+- Add custom headers to client
+- Performance improvements in runner protocol