refactor(backend): update employee schema

- schemas/employee.py: update employee data schemas

server/src/app/schemas/employee.py

@@ -5,6 +5,16 @@ from datetime import date, datetime
 from pydantic import BaseModel, ConfigDict, EmailStr, Field
 
 
+def _parse_optional_date(value: str | None, label: str) -> date | None:
+    if not value:
+        return None
+
+    try:
+        return datetime.strptime(value, "%Y-%m-%d").date()
+    except ValueError as exc:
+        raise ValueError(f"{label}格式必须为 YYYY-MM-DD。") from exc
+
+
 class EmployeeHistoryRead(BaseModel):
     action: str
     owner: str
@@ -96,10 +106,10 @@ class EmployeeCreate(BaseModel):
     role_codes: list[str] = Field(default_factory=lambda: ["user"])
 
     def parsed_birth_date(self) -> date | None:
-        return datetime.strptime(self.birth_date, "%Y-%m-%d").date() if self.birth_date else None
+        return _parse_optional_date(self.birth_date, "出生日期")
 
     def parsed_join_date(self) -> date | None:
-        return datetime.strptime(self.join_date, "%Y-%m-%d").date() if self.join_date else None
+        return _parse_optional_date(self.join_date, "入职日期")
 
 
 class EmployeeUpdate(BaseModel):
@@ -118,7 +128,7 @@ class EmployeeUpdate(BaseModel):
     password: str | None = Field(default=None, min_length=5, max_length=128)
 
     def parsed_birth_date(self) -> date | None:
-        return datetime.strptime(self.birth_date, "%Y-%m-%d").date() if self.birth_date else None
+        return _parse_optional_date(self.birth_date, "出生日期")
 
     def parsed_join_date(self) -> date | None:
-        return datetime.strptime(self.join_date, "%Y-%m-%d").date() if self.join_date else None
+        return _parse_optional_date(self.join_date, "入职日期")