feat: [finance] 계정과목 및 일반전표 API 추가

- AccountCode 모델/서비스/컨트롤러 구현
- JournalEntry, JournalEntryLine 모델 구현
- GeneralJournalEntry 서비스/컨트롤러 구현
- FormRequest 검증 클래스 추가
- finance 라우트 등록
- i18n 메시지 키 추가 (message.php, error.php)

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

app/Models/Tenants/JournalEntryLine.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace App\Models\Tenants;
+
+use App\Traits\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class JournalEntryLine extends Model
+{
+    use BelongsToTenant;
+
+    protected $fillable = [
+        'tenant_id',
+        'journal_entry_id',
+        'line_no',
+        'dc_type',
+        'account_code',
+        'account_name',
+        'trading_partner_id',
+        'trading_partner_name',
+        'debit_amount',
+        'credit_amount',
+        'description',
+    ];
+
+    protected $casts = [
+        'line_no' => 'integer',
+        'debit_amount' => 'integer',
+        'credit_amount' => 'integer',
+        'trading_partner_id' => 'integer',
+    ];
+
+    // DC Type
+    public const DC_DEBIT = 'debit';
+    public const DC_CREDIT = 'credit';
+
+    /**
+     * 전표 관계
+     */
+    public function journalEntry(): BelongsTo
+    {
+        return $this->belongsTo(JournalEntry::class);
+    }
+}