Because OpenWrt is a full Linux, you can install docker (yes, on a Time Capsule – albeit a very stripped-down version), or python3 , or node.js . You could run a local web server from your router. It’s absurd, but absolutely possible.
Custom solutions allow the Time Capsule to continue serving as a Network Attached Storage (NAS) even after macOS stops recognizing it as a Time Machine destination. apple time capsule custom firmware