Fix messaging_channels credentials column: jsonb -> text for encrypted data

PostgreSQL rejects encrypted (base64) strings in jsonb columns. The
encrypted:array cast produces non-JSON output that must be stored as text.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/2024_01_01_000081_fix_messaging_channels_credentials_column.php

+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('messaging_channels', function (Blueprint $table) {
+            $table->text('credentials')->default('')->change();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('messaging_channels', function (Blueprint $table) {
+            $table->jsonb('credentials')->default('{}')->change();
+        });
+    }
+};