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/department_member.py

@@ -0,0 +1,53 @@
+"""
+部門成員 Model
+記錄員工與部門的多對多關係 (純粹組織歸屬，與 RBAC 權限無關)
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, ForeignKey, UniqueConstraint, Index
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class DepartmentMember(Base):
+    """部門成員表"""
+
+    __tablename__ = "tenant_dept_members"
+    __table_args__ = (
+        UniqueConstraint("employee_id", "department_id", name="uq_employee_department"),
+        Index("idx_dept_members_tenant", "tenant_id"),
+        Index("idx_dept_members_employee", "employee_id"),
+        Index("idx_dept_members_department", "department_id"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("tenants.id", ondelete="CASCADE"), nullable=False,
+                       comment="租戶 ID")
+    employee_id = Column(Integer, ForeignKey("tenant_employees.id", ondelete="CASCADE"), nullable=False,
+                         comment="員工 ID")
+    department_id = Column(Integer, ForeignKey("tenant_departments.id", ondelete="CASCADE"), nullable=False,
+                           comment="部門 ID")
+    position = Column(String(100), nullable=True, comment="在該部門的職稱")
+    membership_type = Column(String(50), default="permanent", nullable=False,
+                             comment="成員類型: permanent/temporary/project")
+
+    # 時間記錄（審計追蹤）
+    joined_at = Column(DateTime, default=datetime.utcnow, nullable=False, comment="加入時間")
+    ended_at = Column(DateTime, nullable=True, comment="離開時間（軟刪除）")
+
+    # 審計欄位
+    assigned_by = Column(String(36), nullable=True, comment="分配者 keycloak_user_id")
+    removed_by = Column(String(36), nullable=True, comment="移除者 keycloak_user_id")
+
+    # 通用欄位
+    is_active = Column(Boolean, default=True, nullable=False, comment="是否啟用")
+    edit_by = Column(String(36), nullable=True, comment="最後編輯者 keycloak_user_id")
+    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
+    employee = relationship("Employee", back_populates="department_memberships")
+    department = relationship("Department", back_populates="members")
+
+    def __repr__(self):
+        return f"<DepartmentMember employee_id={self.employee_id} department_id={self.department_id}>"