fix:휴일 년도별 삭제 POST 방식으로 변경

- DELETE /year/{year} → POST /destroy-year 변경
- JavaScript fetch도 POST + JSON body 방식으로 변경

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

routes/web.php

@@ -399,7 +399,7 @@
         Route::get('/list', [HolidayController::class, 'list'])->name('list');
         Route::post('/', [HolidayController::class, 'store'])->name('store');
         Route::post('/bulk', [HolidayController::class, 'bulkStore'])->name('bulk');
-        Route::delete('/year/{year}', [HolidayController::class, 'destroyByYear'])->name('destroy-year');
+        Route::post('/destroy-year', [HolidayController::class, 'destroyByYear'])->name('destroy-year');
         Route::put('/{id}', [HolidayController::class, 'update'])->name('update');
         Route::delete('/{id}', [HolidayController::class, 'destroy'])->name('destroy');
     });