feat: Add admin bulk-mail placeholder variables and template rendering

database/migrations/2023_08_14_221234_create_v2_settings_table.php

@@ -17,7 +17,7 @@ class CreateV2SettingsTable extends Migration
             $table->id();
             $table->string('group')->comment('设置分组')->nullable();
             $table->string('type')->comment('设置类型')->nullable();
-            $table->string('name')->comment('设置名称')->uniqid();
+            $table->string('name')->comment('设置名称')->unique();
             $table->string('value')->comment('设置值')->nullable();
             $table->timestamps();
         });