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/Office/ApprovalNode.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace App\Models\Office;
+
+use App\Models\User;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class ApprovalNode extends Model
+{
+    const UPDATED_AT = null;
+
+    protected $fillable = [
+        'approval_id', 'node_order', 'approver_id', 'action', 'comment', 'acted_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'node_order' => 'integer',
+            'action' => 'integer',
+            'acted_at' => 'datetime',
+        ];
+    }
+
+    public function approval(): BelongsTo
+    {
+        return $this->belongsTo(Approval::class);
+    }
+
+    public function approver(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'approver_id');
+    }
+}