Merge DATUM ngit proposal with preserved catalog entries

Reviewed ngit proposal 15ff5fb9 (pr/datum), head febdda968e. Preserve all existing catalog entries and normalize the new manifest memory limit to supported 512m syntax.

Validation: three configuration preservation tests and strict release catalog drift pass. Live miner shares, payout configuration and reboot acceptance remain separate.

app-catalog/catalog.json

@@ -685,6 +685,21 @@
       "tier": "optional",
       "repoUrl": "https://github.com/bencoin21/justworks.cash",
       "dockerImage": "localhost/archipelago-justworks:0.1.0"
+    },
+    {
+      "id": "datum",
+      "author": "OCEAN contributors",
+      "requires": [
+        "bitcoin-knots"
+      ],
+      "title": "DATUM",
+      "version": "0.4.1-beta.1",
+      "description": "Build Bitcoin mining templates on your own node and connect your miners to OCEAN through DATUM.",
+      "dockerImage": "localhost/archipelago-datum:0.4.1-beta.1",
+      "category": "bitcoin",
+      "tier": "optional",
+      "icon": "/assets/img/app-icons/datum.svg",
+      "repoUrl": "https://github.com/OCEAN-xyz/datum_gateway"
     }
   ]
 }