feat(tailscale): enable mesh networking across NixOS hosts

- enable Tailscale on Bob, Brain, Jeeves, and Rhapsody
- configure the Jeeves binary cache through Tailscale MagicDNS
- trust the existing workshop cache signing key

systems/rhapsody-in-green/default.nix

@@ -8,6 +8,7 @@
     "${inputs.self}/common/optional/steam.nix"
     "${inputs.self}/common/optional/syncthing_base.nix"
     "${inputs.self}/common/optional/systemd-boot.nix"
+    "${inputs.self}/common/optional/tailscale.nix"
     "${inputs.self}/common/optional/yubikey.nix"
     "${inputs.self}/common/optional/zerotier.nix"
     "${inputs.self}/common/optional/zfs"