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/baserow/module.json

@@ -36,6 +36,16 @@
       "why": "the Baserow web UI and REST API, served by the image's own Caddy; a public name is the route's"
     }
   ],
+  "data": {
+    "own": [
+      {
+        "id": "data",
+        "path": "${dir:data}",
+        "class": "valuable",
+        "why": "uploaded files and media; the tables are in the database"
+      }
+    ]
+  },
   "resources": [
     {
       "id": "mesh-state",