fix(ha): use stable by-id disk path for DRBD instead of /dev/sd*

/dev/sda and /dev/sdb are assigned by the OS based on Proxmox disk-add
order, which is not consistent across VMs. Use the SCSI controller path
instead — drive-scsi1 is always the dedicated data disk on all HA nodes
regardless of which sda/sdb it gets assigned to.

variables.nix: replace per-node haServer{1,2}DrbdDisk with a single
haServerDrbdDisk using /dev/disk/by-id/scsi-0QEMU_QEMU_HARDDISK_drive-scsi1.

cluster-config.nix: revert to shared volume block (per-node block was
needed for /dev/sd* but the by-id path is identical on both nodes).

cluster-init.sh:
- single DRBD_DISK variable (matching haServerDrbdDisk)
- robust /etc/drbd.conf patch: NixOS manages this file as a symlink to
  a read-only Nix store path; cp --remove-destination breaks the symlink
  before sed -i so the edit actually takes effect
- scp helper script to NODE2 rather than bash -c over SSH to avoid
  quoting complexity

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

modules/ha/cluster-config.nix

@@ -68,22 +68,18 @@
       }
 
       resource ha-data {
+        volume 0 {
+          device    /dev/drbd0;
+          disk      ${vars.haServerDrbdDisk};
+          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;
-          }
         }
       }
     '';