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

app/Models/KnowledgeBase.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\SoftDeletes;
+
+class KnowledgeBase extends Model
+{
+    use HasFactory, SoftDeletes;
+
+    /**
+     * 可批量赋值的属性
+     *
+     * @var array<string>
+     */
+    protected $fillable = [
+        'name',
+        'description',
+        'status',
+    ];
+
+    /**
+     * 获取关联的终端
+     *
+     * @return \Illuminate\Database\Eloquent\Relations\BelongsToMany
+     */
+    public function terminals()
+    {
+        return $this->belongsToMany(Terminal::class, 'terminal_knowledge_bases')
+                    ->withPivot('priority')
+                    ->withTimestamps()
+                    ->orderBy('priority');
+    }
+}