Declare every module's data; the backup holder measures it (hq ADR 0233)

Backup lines are derived from each module's data section instead of written by hand; the holder
measures declared items, reads the array under them, and deletes a retired item only after a last
restore point; the Go providers say each held consumer's size so an empty replacement is seen.

modules/mosquitto/module.json

@@ -53,6 +53,23 @@
       "why": "the same broker over MQTT-on-WebSockets, for browser clients"
     }
   ],
+  "data": {
+    "own": [
+      {
+        "id": "data",
+        "path": "${dir:data}",
+        "class": "rebuildable",
+        "why": "retained messages and sessions; devices publish them again"
+      }
+    ],
+    "consumers": {
+      "mqtt-topic": {
+        "class": "rebuildable",
+        "in": "data",
+        "why": "retained messages are published again by the devices"
+      }
+    }
+  },
   "resources": [
     {
       "id": "mesh-state",