fix(ha): per-node DRBD disk assignment (NODE2 data disk is /dev/sda not /dev/sdb)

Proxmox VM disk ordering differs between the two HA nodes:
  ha-server-1: sda=OS (50G), sdb=DRBD data (32G)
  ha-server-2: sda=DRBD data (32G), sdb=OS (50G)

The DRBD resource config was using a shared disk=/dev/sdb which targeted
the OS disk on ha-server-2, causing drbdmeta and drbdadm up to operate
on the mounted root filesystem (hence "Device or resource busy").

Changes:
- variables.nix: add haServer1DrbdDisk/haServer2DrbdDisk
- cluster-config.nix: move volume block inside per-host on{} sections
  so each node uses the correct backing disk
- cluster-init.sh: use NODE1_DRBD_DISK/NODE2_DRBD_DISK variables;
  add runtime check that patches /etc/drbd.d/*.res on the running nodes
  if the deployed config points to the wrong disk (workaround for VMs
  built before this fix; redeploy with --force-rebuild to make permanent)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/ha/cluster-config.nix

@@ -68,18 +68,22 @@
       }
 
       resource ha-data {
-        volume 0 {
-          device    /dev/drbd0;
-          disk      /dev/sdb;
-          meta-disk internal;
-        }
-
         on ${vars.haServer1Host} {
           address ${vars.haServer1StorageIp}:${toString vars.ports.haServerDrbd};
+          volume 0 {
+            device    /dev/drbd0;
+            disk      ${vars.haServer1DrbdDisk};
+            meta-disk internal;
+          }
         }
 
         on ${vars.haServer2Host} {
           address ${vars.haServer2StorageIp}:${toString vars.ports.haServerDrbd};
+          volume 0 {
+            device    /dev/drbd0;
+            disk      ${vars.haServer2DrbdDisk};
+            meta-disk internal;
+          }
         }
       }
     '';