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>

docs/plans/nix-cache-reprovision.md

@@ -12,6 +12,7 @@ Reprovision `nix-cache01` using the OpenTofu workflow, and improve the build/cac
 **Phase 2: NATS Build Triggering** - COMPLETE
 **Phase 3: Safe Flake Update Workflow** - NOT STARTED
 **Phase 4: Complete Migration** - COMPLETE
+**Phase 5: Scheduled Builds** - COMPLETE
 
 ## Completed Work
 
@@ -103,24 +104,20 @@ The `homelab-deploy` tool was extended with a builder mode:
    - Removed from `flake.nix`
    - Deleted VM from Proxmox
 
-### Phase 5: Scheduled Builds (Optional)
+### Phase 5: Scheduled Builds ✅
 
-Add a systemd timer on nix-cache02 to trigger periodic builds via NATS:
+Implemented a systemd timer on nix-cache02 that triggers builds every 2 hours:
 
-```nix
-systemd.services.scheduled-build = {
-  script = ''
-    homelab-deploy build nixos-servers --all
-    homelab-deploy build nixos --all
-  '';
-};
-systemd.timers.scheduled-build = {
-  wantedBy = [ "timers.target" ];
-  timerConfig.OnCalendar = "*-*-* *:30:00";
-};
-```
+- **Timer**: `scheduled-build.timer` runs every 2 hours with 5m random jitter
+- **Service**: `scheduled-build.service` calls `homelab-deploy build` for both repos
+- **Authentication**: Dedicated scheduler NKey stored in Vault
+- **NATS user**: Added to DEPLOY account with publish `build.>` and subscribe `build.responses.>`
 
-Or trigger builds from CI after merges to master.
+Files:
+- `hosts/nix-cache02/scheduler.nix` - Timer and service configuration
+- `services/nats/default.nix` - Scheduler NATS user
+- `terraform/vault/secrets.tf` - Scheduler NKey secret
+- `terraform/vault/variables.tf` - Variable for scheduler NKey
 
 ## Resolved Questions