fix: 특수 일정 ID에 연도 포함

- 생일: birthday-{year}-{name_en} (예: birthday-2025-saerom)
- 데뷔: debut-{year} (예: debut-2018)
- 주년: anniversary-{year} (예: anniversary-2026)

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

backend/src/services/schedule.js

@@ -334,7 +334,7 @@ export async function getMonthlySchedules(db, year, month) {
     const birthdayDate = new Date(year, birthDate.getMonth(), birthDate.getDate());
 
     schedules.push({
-      id: `birthday-${member.id}`,
+      id: `birthday-${year}-${member.name_en.toLowerCase()}`,
       title: `HAPPY ${member.name_en} DAY`,
       date: birthdayDate.toISOString().split('T')[0],
       time: null,
@@ -357,7 +357,7 @@ export async function getMonthlySchedules(db, year, month) {
       if (year === debutYear) {
         // 데뷔 당일
         schedules.push({
-          id: 'debut',
+          id: `debut-${year}`,
           title: '프로미스나인 데뷔',
           date: debutDate.toISOString().split('T')[0],
           time: null,
@@ -369,7 +369,7 @@ export async function getMonthlySchedules(db, year, month) {
       } else {
         // N주년
         schedules.push({
-          id: `anniversary-${anniversaryYear}`,
+          id: `anniversary-${year}`,
           title: `프로미스나인 데뷔 ${anniversaryYear}주년`,
           date: debutDate.toISOString().split('T')[0],
           time: null,