feat: HR Portal - Complete Multi-Tenant System with Redis Session Storage

Major Features:
- ✅ Multi-tenant architecture (tenant isolation)
- ✅ Employee CRUD with lifecycle management (onboarding/offboarding)
- ✅ Department tree structure with email domain management
- ✅ Company info management (single-record editing)
- ✅ System functions CRUD (permission management)
- ✅ Email account management (multi-account per employee)
- ✅ Keycloak SSO integration (auth.lab.taipei)
- ✅ Redis session storage (10.1.0.254:6379)
  - Solves Cookie 4KB limitation
  - Cross-system session sharing
  - Sliding expiration (8 hours)
  - Automatic token refresh

Technical Stack:
Backend:
- FastAPI + SQLAlchemy
- PostgreSQL 16 (10.1.0.20:5433)
- Keycloak Admin API integration
- Docker Mailserver integration (SSH)
- Alembic migrations

Frontend:
- Next.js 14 (App Router)
- NextAuth 4 with Keycloak Provider
- Redis session storage (ioredis)
- Tailwind CSS

Infrastructure:
- Redis 7 (10.1.0.254:6379) - Session + Cache
- Keycloak 26.1.0 (auth.lab.taipei)
- Docker Mailserver (10.1.0.254)

Architecture Highlights:
- Session管理由 Keycloak + Redis 統一控制
- 支援多系統 (HR/WebMail/Calendar/Drive/Office) 共享 session
- Token 自動刷新,異質服務整合
- 未來可無縫遷移到雲端

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

backend/app/models/employee.py

@@ -0,0 +1,85 @@
+"""
+員工基本資料 Model
+"""
+from datetime import datetime, date
+from sqlalchemy import Column, Integer, String, Boolean, Date, DateTime, Enum, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import relationship
+import enum
+
+from app.db.base import Base
+
+
+class EmployeeStatus(str, enum.Enum):
+    """員工狀態"""
+    ACTIVE = "active"
+    INACTIVE = "inactive"
+    TERMINATED = "terminated"
+
+
+class Employee(Base):
+    """員工基本資料表"""
+
+    __tablename__ = "tenant_employees"
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "employee_id", name="uq_tenant_employee_id"),
+        UniqueConstraint("tenant_id", "seq_no", name="uq_tenant_seq_no"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("tenants.id", ondelete="CASCADE"), nullable=False, index=True, comment="租戶 ID")
+    seq_no = Column(Integer, nullable=False, comment="租戶內序號 (自動從1開始)")
+    employee_id = Column(String(20), nullable=False, index=True, comment="員工編號 (EMP001, 租戶內唯一，永久不變)")
+    keycloak_user_id = Column(String(36), unique=True, nullable=True, index=True,
+                              comment="Keycloak User UUID (唯一 SSO 識別碼，永久不變，一個員工只有一個)")
+    username_base = Column(String(50), unique=True, nullable=False, index=True, comment="基礎帳號名稱 (全系統唯一)")
+    legal_name = Column(String(100), nullable=False, comment="法定姓名")
+    english_name = Column(String(100), comment="英文名稱")
+    phone = Column(String(20), comment="電話")
+    mobile = Column(String(20), comment="手機")
+    hire_date = Column(Date, nullable=False, comment="到職日期")
+    status = Column(
+        String(20),
+        default=EmployeeStatus.ACTIVE,
+        nullable=False,
+        comment="狀態 (active/inactive/terminated)"
+    )
+
+    # 通用欄位
+    is_active = Column(Boolean, default=True, nullable=False, comment="是否啟用")
+    edit_by = Column(String(100), nullable=True, comment="最後編輯者")
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False, comment="建立時間")
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False, comment="更新時間")
+
+    # 關聯
+    tenant = relationship("Tenant", back_populates="employees")
+    department_memberships = relationship(
+        "DepartmentMember",
+        back_populates="employee",
+        cascade="all, delete-orphan",
+        lazy="selectin"
+    )
+    email_accounts = relationship(
+        "EmailAccount",
+        back_populates="employee",
+        cascade="all, delete-orphan",
+        lazy="selectin"
+    )
+    permissions = relationship(
+        "Permission",
+        foreign_keys="Permission.employee_id",
+        back_populates="employee",
+        cascade="all, delete-orphan",
+        lazy="selectin"
+    )
+    network_drive = relationship(
+        "NetworkDrive",
+        back_populates="employee",
+        uselist=False,
+        cascade="all, delete-orphan",
+        lazy="selectin"
+    )
+
+    def __repr__(self):
+        return f"<Employee {self.employee_id} - {self.legal_name}>"
+
+    # is_active 已改為資料庫欄位，移除 @property