feat: add Leave model with auto-migration

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

backend/app/models/leave.py

@@ -0,0 +1,30 @@
+import uuid
+from datetime import date, datetime
+from sqlalchemy import String, Date, DateTime, ForeignKey, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+from app.database import Base
+
+
+class Leave(Base):
+    __tablename__ = "leaves"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    manager_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id"), index=True
+    )
+    leave_type: Mapped[str] = mapped_column(String(20), default="事假")
+    start_date: Mapped[date] = mapped_column(Date)
+    end_date: Mapped[date] = mapped_column(Date)
+    reason: Mapped[str] = mapped_column(String(500), default="")
+    submitted_by: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id")
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )