feat: 第五阶段办公协同/统计报表/知识库模块

- 新增3个migration(9张表): office(6表)/report(1表)/kb(2表)
- 新增9个Model + 9个Controller
- 新增43条API路由(总计305条)
- 新增3个前端API模块 + 10个管理页面
- 迁移运行通过, vite build验证通过

backend/app/Models/Kb/KbCategory.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Models\Kb;
+
+use App\Traits\BelongsToStore;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class KbCategory extends Model
+{
+    use BelongsToStore;
+
+    protected $fillable = [
+        'store_id', 'parent_id', 'name', 'sort', 'status',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'sort' => 'integer',
+            'status' => 'integer',
+        ];
+    }
+
+    public function children(): HasMany
+    {
+        return $this->hasMany(self::class, 'parent_id');
+    }
+
+    public function parent(): BelongsTo
+    {
+        return $this->belongsTo(self::class, 'parent_id');
+    }
+
+    public function articles(): HasMany
+    {
+        return $this->hasMany(KbArticle::class, 'category_id');
+    }
+}