feat(阶段三): 添加知识库模型和迁移

- 创建 knowledge_bases 表迁移
- 创建 KnowledgeBase 模型
- 创建 KnowledgeBaseFactory 工厂类
- 支持与终端的多对多关联关系

database/migrations/2026_03_09_022236_create_knowledge_bases_table.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('knowledge_bases', function (Blueprint $table) {
+            $table->id();
+            $table->string('name')->comment('知识库名称');
+            $table->text('description')->nullable()->comment('知识库描述');
+            $table->string('status')->default('active')->comment('状态');
+            $table->timestamps();
+            $table->softDeletes();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('knowledge_bases');
+    }
+};