caveman: set redis backup schedule

hosts/caveman/default.nix

@@ -22,6 +22,14 @@
   '';
 
   services = {
+    # Override default backup schedule to reduce I/O
+    redis.servers.caveman.save = [
+      # Every 2h if at least 1 entry changed
+      [ 7200 1 ]
+      # Every 30min if at least 10000 entries changed
+      [ 1800 10000 ]
+    ];
+
     caveman = {
       # leave 4 GB for caveman services
       redis.maxmemory = (config.microvm.mem - 4) * 1024 * 1024;