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>

variables.nix

@@ -108,11 +108,11 @@
   haStoragePrefixLength = 29; # storage subnet prefix length (/29)
   haStorageRoot = "/srv/ha-data"; # XFS-over-DRBD mount point on the Active node
   haIscsiIqn = "iqn.2026-01.home.sweet:ha-storage";
-  # DRBD backing device on each node. Disk ordering can differ between Proxmox
-  # VMs depending on the order disks were added; these must match the actual
-  # block device that is NOT the OS disk on each node (verify with lsblk).
-  haServer1DrbdDisk = "/dev/sdb"; # data disk on ha-server-1 (OS disk is /dev/sda)
-  haServer2DrbdDisk = "/dev/sda"; # data disk on ha-server-2 (OS disk is /dev/sdb)
+  # DRBD backing disk — identified by SCSI controller path so it resolves to the
+  # correct block device regardless of OS-level naming (sda vs sdb can differ
+  # between Proxmox VMs depending on disk-add order). drive-scsi1 is always the
+  # dedicated data disk on all HA nodes; drive-scsi0 is the OS disk.
+  haServerDrbdDisk = "/dev/disk/by-id/scsi-0QEMU_QEMU_HARDDISK_drive-scsi1";
 
   # Storage
   storageRoot = "/tank"; # ZFS pool root on `server`