feat: [payroll] 전월 급여 복사 등록 기능 추가

- PayrollService에 copyFromPreviousMonth() 메서드 추가
- PayrollController에 copyFromPrevious() 액션 추가
- 전월 지급/공제 금액을 그대로 복사 (요율 재계산 없음)
- 이미 존재하는 사원/연월은 스킵 처리

app/Http/Controllers/Api/Admin/HR/PayrollController.php

@@ -271,6 +271,45 @@ public function pay(Request $request, int $id): JsonResponse
         }
     }
 
+    /**
+     * 전월 급여 복사 등록
+     */
+    public function copyFromPrevious(Request $request): JsonResponse
+    {
+        $validated = $request->validate([
+            'pay_year' => 'required|integer|min:2020|max:2100',
+            'pay_month' => 'required|integer|min:1|max:12',
+        ]);
+
+        try {
+            $result = $this->payrollService->copyFromPreviousMonth($validated['pay_year'], $validated['pay_month']);
+
+            if (! empty($result['no_previous'])) {
+                $prevYear = $validated['pay_month'] === 1 ? $validated['pay_year'] - 1 : $validated['pay_year'];
+                $prevMonth = $validated['pay_month'] === 1 ? 12 : $validated['pay_month'] - 1;
+
+                return response()->json([
+                    'success' => false,
+                    'message' => "{$prevYear}년 {$prevMonth}월 급여 데이터가 없습니다.",
+                ], 422);
+            }
+
+            return response()->json([
+                'success' => true,
+                'message' => "전월 복사 완료: {$result['created']}건 생성, {$result['skipped']}건 건너뜀 (이미 존재).",
+                'data' => $result,
+            ]);
+        } catch (\Throwable $e) {
+            report($e);
+
+            return response()->json([
+                'success' => false,
+                'message' => '전월 복사 중 오류가 발생했습니다.',
+                'error' => config('app.debug') ? $e->getMessage() : null,
+            ], 500);
+        }
+    }
+
     /**
      * 일괄 생성
      */

app/Services/HR/PayrollService.php

@@ -302,6 +302,74 @@ public function payPayroll(int $id): ?Payroll
         return $payroll->fresh(['user']);
     }
 
+    /**
+     * 전월 급여 복사 등록
+     */
+    public function copyFromPreviousMonth(int $year, int $month): array
+    {
+        $tenantId = session('selected_tenant_id', 1);
+
+        // 전월 계산 (1월 → 전년 12월)
+        $prevYear = $month === 1 ? $year - 1 : $year;
+        $prevMonth = $month === 1 ? 12 : $month - 1;
+
+        $previousPayrolls = Payroll::query()
+            ->forTenant($tenantId)
+            ->forPeriod($prevYear, $prevMonth)
+            ->get();
+
+        if ($previousPayrolls->isEmpty()) {
+            return ['created' => 0, 'skipped' => 0, 'no_previous' => true];
+        }
+
+        $created = 0;
+        $skipped = 0;
+
+        DB::transaction(function () use ($previousPayrolls, $tenantId, $year, $month, &$created, &$skipped) {
+            foreach ($previousPayrolls as $prev) {
+                $exists = Payroll::query()
+                    ->where('tenant_id', $tenantId)
+                    ->where('user_id', $prev->user_id)
+                    ->forPeriod($year, $month)
+                    ->exists();
+
+                if ($exists) {
+                    $skipped++;
+
+                    continue;
+                }
+
+                Payroll::create([
+                    'tenant_id' => $tenantId,
+                    'user_id' => $prev->user_id,
+                    'pay_year' => $year,
+                    'pay_month' => $month,
+                    'base_salary' => $prev->base_salary,
+                    'overtime_pay' => $prev->overtime_pay,
+                    'bonus' => $prev->bonus,
+                    'allowances' => $prev->allowances,
+                    'gross_salary' => $prev->gross_salary,
+                    'income_tax' => $prev->income_tax,
+                    'resident_tax' => $prev->resident_tax,
+                    'health_insurance' => $prev->health_insurance,
+                    'long_term_care' => $prev->long_term_care,
+                    'pension' => $prev->pension,
+                    'employment_insurance' => $prev->employment_insurance,
+                    'deductions' => $prev->deductions,
+                    'total_deductions' => $prev->total_deductions,
+                    'net_salary' => $prev->net_salary,
+                    'status' => 'draft',
+                    'created_by' => auth()->id(),
+                    'updated_by' => auth()->id(),
+                ]);
+
+                $created++;
+            }
+        });
+
+        return ['created' => $created, 'skipped' => $skipped];
+    }
+
     /**
      * 일괄 생성 (재직 사원 전체)
      */