Fix payload conversion in migrate_to_persist_sqlite.py

Closes #3492. Thanks to Marc Haesen.

ChangeLog.txt

@@ -1,10 +1,14 @@
 2.1.3 - 2026-02-xx
 ==================
 
-Broker:
+# Broker:
 - Fix MOSQ_EVT_DISCONNECT being called before MOSQ_EVT_ACL_CHECK for the will
   of that client. Closes #3487.
 
+Plugins
+- Fix migrate_to_persist_sqlite.py not base64 decoding message payloads when
+  migrating. Closes #3492.
+
 
 2.1.2 - 2026-02-09
 ==================

plugins/persist-sqlite/migrate_to_persist_sqlite.py

@@ -4,6 +4,7 @@ Migration script to migrate from Snapshot persistence to Persist SQLite Plugin.
 """
 
 import argparse
+import base64
 import json
 import shutil
 import sqlite3
@@ -228,6 +229,9 @@ class SQLite3Persistence:
     def __add_base_msgs(self, base_msgs: list[dict]):
         for base_msg in base_msgs:
             self.__store_id_topic_map.update({base_msg["storeid"]: base_msg["topic"]})
+
+            payload = base64.b64decode(base_msg["payload"]) if "payload" in base_msg else None
+
             self.__cursor.execute(
                 "INSERT INTO base_msgs "
                 "(store_id, expiry_time, topic, payload, source_id, source_username, "
@@ -237,10 +241,10 @@ class SQLite3Persistence:
                     base_msg["storeid"],
                     base_msg["expiry-time"],
                     base_msg["topic"],
-                    base_msg["payload"] if "payload" in base_msg else None,
+                    payload if "payload" in base_msg else None,
                     base_msg["clientid"] if "clientid" in base_msg else None,
                     base_msg["username"] if "username" in base_msg else None,
-                    len(base_msg["payload"]) if "username" in base_msg else 0,
+                    len(payload) if "username" in base_msg else 0,
                     base_msg["source-mid"],
                     base_msg["source-port"],
                     base_msg["qos"],