restic: take the retention from the machine's settings, so a disk can keep more or fewer restore points without a new build (novox/hq issue 416)

The keep-daily, keep-weekly and keep-monthly preferences default to
today's 14, 8 and 6. The night reads them from a file in its state and
refuses anything but a count from 1 to 9999: it then thins nothing and
says which setting is wrong, never handing a raw value to restic.

modules/restic/module.json

@@ -32,6 +32,23 @@
       }
     ]
   },
+  "settings": {
+    "keep-daily": {
+      "kind": "preference",
+      "default": 14,
+      "why": "two weeks of nightly restore points undo a mistake noticed within a fortnight; a count from 1 to 9999, read by the night, which thins nothing while it does not read (novox/hq issue 416)"
+    },
+    "keep-weekly": {
+      "kind": "preference",
+      "default": 8,
+      "why": "two months of weekly restore points behind the dailies; a count from 1 to 9999, refused by the night like keep-daily"
+    },
+    "keep-monthly": {
+      "kind": "preference",
+      "default": 6,
+      "why": "half a year of monthly restore points behind the weeklies; a count from 1 to 9999, refused by the night like keep-daily"
+    }
+  },
   "resources": [
     {
       "id": "state",
@@ -58,6 +75,14 @@
       "mode": "0600",
       "content": "# The data the modules on this machine declare, composed by the mesh (novox/hq ADR 0233). Do not edit.\n${contribution:node-backup:data}"
     },
+    {
+      "id": "retention",
+      "type": "file",
+      "path": "${dir:state}/retention.conf",
+      "mode": "0600",
+      "trusted": true,
+      "content": "# How many restore points the night keeps, written by the mesh from this machine's settings keep-daily,\n# keep-weekly and keep-monthly (novox/hq issue 416). Replaced on every push; change the settings, never this file.\ndaily=${setting:keep-daily}\nweekly=${setting:keep-weekly}\nmonthly=${setting:keep-monthly}\n"
+    },
     {
       "id": "tool",
       "type": "package",