feat: Phase 6.1 악성채권 추심관리 API 구현

- 테이블 3개: bad_debts, bad_debt_documents, bad_debt_memos
- 모델 3개: BadDebt, BadDebtDocument, BadDebtMemo
- BadDebtService: CRUD, 요약 통계, 서류/메모 관리
- API 엔드포인트 11개 (목록, 등록, 상세, 수정, 삭제, 토글, 서류/메모 CRUD)
- Swagger 문서 작성 완료

app/Http/Requests/V1/BadDebt/StoreBadDebtDocumentRequest.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace App\Http\Requests\V1\BadDebt;
+
+use App\Models\BadDebts\BadDebtDocument;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class StoreBadDebtDocumentRequest extends FormRequest
+{
+    /**
+     * Determine if the user is authorized to make this request.
+     */
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * Get the validation rules that apply to the request.
+     *
+     * @return array<string, \Illuminate\Contracts\Validation\ValidationRule|array<mixed>|string>
+     */
+    public function rules(): array
+    {
+        return [
+            'document_type' => ['required', 'string', Rule::in(array_keys(BadDebtDocument::DOCUMENT_TYPES))],
+            'file_id' => ['required', 'integer', 'exists:files,id'],
+        ];
+    }
+
+    /**
+     * Get custom messages for validator errors.
+     *
+     * @return array<string, string>
+     */
+    public function messages(): array
+    {
+        return [
+            'document_type.required' => __('validation.required', ['attribute' => '서류유형']),
+            'document_type.in' => __('validation.in', ['attribute' => '서류유형']),
+            'file_id.required' => __('validation.required', ['attribute' => '파일']),
+            'file_id.exists' => __('validation.exists', ['attribute' => '파일']),
+        ];
+    }
+}