Standardize system disk on LVM-on-LUKS for snapshot support.

Add dedicated parity disk and correct data disk mountpoints.
Resolve various Nix syntax errors in disk templates.
Set data disk filesystem to XFS for better large-file performance.

config-files/sops-nix/.sops.yaml

@@ -0,0 +1,9 @@
+# .sops.yaml
+
+keys:
+  - &primary $SOPS_PUBLIC_KEY
+creation_rules:
+  - path_regex: secrets/secrets.yaml$
+    key_groups:
+    - age:
+      - *primary