feat: [receiving] 입고 성적서 파일 연결 기능 추가

- receivings 테이블에 certificate_file_id 컬럼 추가 (마이그레이션)
- Receiving 모델에 certificateFile 관계 및 fillable/casts 추가
- Store/Update Request에 certificate_file_id 검증 규칙 추가
- ReceivingService index/show에 certificateFile eager loading 추가
- store/update 시 certificate_file_id 저장 처리

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/2026_03_11_100000_add_certificate_file_id_to_receivings_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
+{
+    public function up(): void
+    {
+        Schema::table('receivings', function (Blueprint $table) {
+            $table->unsignedBigInteger('certificate_file_id')
+                ->nullable()
+                ->after('options')
+                ->comment('업체 제공 성적서 파일 ID');
+
+            $table->foreign('certificate_file_id')
+                ->references('id')
+                ->on('files')
+                ->nullOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('receivings', function (Blueprint $table) {
+            $table->dropForeign(['certificate_file_id']);
+            $table->dropColumn('certificate_file_id');
+        });
+    }
+};