nix-cache02: add scheduled builds timer

Add a systemd timer that triggers builds for all hosts every 2 hours
via NATS, keeping the binary cache warm.

- Add scheduler.nix with timer (every 2h) and oneshot service
- Add scheduler NATS user to DEPLOY account
- Add Vault secret and variable for scheduler NKey
- Increase nix-cache02 memory from 16GB to 20GB

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

terraform/vms.tf

@@ -89,7 +89,7 @@ locals {
     "nix-cache02" = {
       ip        = "10.69.13.25/24"
       cpu_cores = 8
-      memory    = 16384
+      memory    = 20480
       disk_size = "200G"
       vault_wrapped_token = "s.C5EuHFyULACEqZgsLqMC3cJB"
     }