feat: 연차 관리 기능 추가 (AnnualLeave 엔티티, DAO, Repository)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

app/src/main/java/com/example/shiftalarm/AppDatabase.kt

@@ -3,7 +3,7 @@ package com.example.shiftalarm
 import android.content.Context
 import androidx.room.*
 
-@Database(entities = [ShiftOverride::class, DailyMemo::class, CustomAlarm::class], version = 3, exportSchema = false)
+@Database(entities = [ShiftOverride::class, DailyMemo::class, CustomAlarm::class, AnnualLeave::class], version = 4, exportSchema = false)
 abstract class AppDatabase : RoomDatabase() {
     abstract fun shiftDao(): ShiftDao