Feature request: track stable-targeted PRs (like backports) against stable branches

Example: nixos/nixpkgs#414871 Expected behaviour: https://nixpk.gs/pr-tracker.html?pr=414871 Here stable branches are displayed. Actual behaviour: ```console $ nick 414871 [414871] [Backport staging-25.05] cpython: 3.13.3 -> 3.13.4; 3.12.10 -> 3.12.11; 3.11.12 -> 3.11.13; 3.10.17 -> 3.10.18 Merged 8 days 13 hours 21 minutes and 44 seconds ago (2025-06-14T02:35:11+00:00), 6 days 2 hours 47 minutes and 48 seconds after creation. master: 🚫 staging: 🚫 staging-next: 🚫 nixpkgs-unstable: 🚫 nixos-unstable-small: 🚫 nixos-unstable: 🚫 ```