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>

services/nats/default.nix

@@ -105,6 +105,14 @@
                 publish = [ "build.responses.>" ];
               };
             }
+            # Scheduler (publishes build requests, subscribes to responses)
+            {
+              nkey = "UDQ5SFEGDM66AQGLK7KQDW6ZOC2QCXE2P6EJQ6VPBSR2CRCABPOVWRI4";
+              permissions = {
+                publish = [ "build.>" ];
+                subscribe = [ "build.responses.>" ];
+              };
+            }
           ];
         };
       };