feat: per-host darman passwords via sops; mercury sops; AdGuard pw via UI

- per-host darman_password (distinct hash) in secrets/{jupiter,vps,mercury}.yaml
  -> hashedPasswordFile; different console password per host (ssh still key-only)
- mercury: dedicated age key (on boot partition post-flash), sops-nix wired
- AdGuard: module has no secret hook + writable config -> mutableSettings=true,
  admin password set via web setup on first boot (never in repo/store)

secrets/mercury.yaml

@@ -0,0 +1,25 @@
+darman_password: ENC[AES256_GCM,data:HzFkwGIyQL1sbPegJq970n3VASkaO/iY1t3fjodqk6FSoGX+4zlVoZfNAuTeiyAwv+0bZHhJoK+cAPKp5HMqsnhXJrhJUV7KVA3beyBXPPg8IpXi4X4maKv9KLIiWi2BJQuE421LUjb/2A==,iv:dcCD3aaEfVfPBh0jwFZkmNz0L/ZfRzMROJIHXd6pkG8=,tag:5vRhPo+fE28HrZPp6ZJ1MQ==,type:str]
+sops:
+    age:
+        - enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBHMXdtQ2E3SENhY2RKWGx1
+            bWVPUUd6K2gwbnRjbjVZZnIvSkJpaW1kNXhNCmF5KzFZdDZoVjcybTIzeEltNk1X
+            QXg4bFJmQkhsUkVpQ1pEVHVUMmN2OWcKLS0tIExKdGwzWTJDbEd4b3V6SlIvSXo0
+            SDZMTXYyZThQamN1YUt2ODRhdm1kdEEK8wZbc4NCgWYJ4tZtq2UxATleAhJxor5b
+            H0Fjcz+V0CesaF1sxR1M1gUe7uXrhdNdNgZvr+aF/+jsKHq+thbelw==
+            -----END AGE ENCRYPTED FILE-----
+          recipient: age1cekcqyf7073fsytcjxaa9dr9zwkmn4vjg36rv2tgxdglzfv4jvxqvcj6z2
+        - enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBsNTJkd29HTWdFc1ZhTDFN
+            d2paTy91aGNNSlBWUDdqY255MzN1QUlibldJCmdHY0FVMU1uV2lRUGp0MjVFYjVO
+            RWNsMk0wU2oxaXdlS25TSHJIQnhxMXMKLS0tIG1GRjVSMEgyY2RYZDRSTllVd3g5
+            UGFjMERZTHhuWGpFVUE4TVIyYlA4WmMKcinqjjAaR3hJRLJepZPxunQz9WBvmP6+
+            ZsBrZS0j0B3MhTI+6gUWB82xozgmnh95w+SXSSZZvoss8RuicE5ysA==
+            -----END AGE ENCRYPTED FILE-----
+          recipient: age1cpty7zrgnn6l97upq00w5wa8zcvnkxkdt2jvhlj97jh83exure4slha43t
+    lastmodified: "2026-07-13T18:50:12Z"
+    mac: ENC[AES256_GCM,data:8e1VHvwPzI97toTrkbT53ky8OV8Jepyn5yhEHq87wcDP4cywZTm6WbJsqJNgvYdwwxIgaLK7WyausaFfnqIBRFSxauhmTD7mjXKwGFAvSyeyFL3RYbdTc3+GInTE8gF8oBQ7OGIREW6pcDFswzhmcTKbJ01V+vJ2sgxnvifNe7E=,iv:pmLsvkPro4aw//UdldKErbYfVmlK+xb2/4CbN0I6gQQ=,tag:sy9wBkTYIdYrlJ8U4TdJIw==,type:str]
+    unencrypted_suffix: _unencrypted
+    version: 3.13.1