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/Entities.kt

@@ -1,6 +1,7 @@
 package com.example.shiftalarm
 
-import androidx.room.*
+import androidx.room.Entity
+import androidx.room.PrimaryKey
 
 @Entity(tableName = "shift_overrides", primaryKeys = ["factory", "team", "date"])
 data class ShiftOverride(
@@ -28,3 +29,12 @@ data class CustomAlarm(
     val snoozeInterval: Int = 5,
     val snoozeRepeat: Int = 3
 )
+
+@Entity(tableName = "annual_leave")
+data class AnnualLeave(
+    @PrimaryKey
+    val id: Int = 1, // Single row for app-wide annual leave
+    val totalDays: Float, // 총 연차 (1~25)
+    val remainingDays: Float, // 남은 연차
+    val updatedAt: Long = System.currentTimeMillis()
+)