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.py

@@ -0,0 +1,74 @@
+"""
+部門 Model
+統一樹狀部門結構:
+- depth=0, parent_id=NULL: 第一層部門 (原事業部)，可設定 email_domain
+- depth=1+: 子部門，繼承上層 email_domain
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, Text, ForeignKey, UniqueConstraint, Index
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class Department(Base):
+    """部門表 (統一樹狀結構)"""
+
+    __tablename__ = "tenant_departments"
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "seq_no", name="uq_tenant_dept_seq"),
+        UniqueConstraint("tenant_id", "parent_id", "code", name="uq_tenant_parent_dept_code"),
+        Index("idx_dept_tenant_id", "tenant_id"),
+        Index("idx_departments_parent", "parent_id"),
+        Index("idx_departments_depth", "depth"),
+    )
+
+    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 (觸發器自動生成)")
+    parent_id = Column(Integer, ForeignKey("tenant_departments.id", ondelete="CASCADE"), nullable=True,
+                       comment="上層部門 ID (NULL=第一層，即原事業部)")
+    code = Column(String(20), nullable=False, comment="部門代碼 (同層內唯一)")
+    name = Column(String(100), nullable=False, comment="部門名稱")
+    name_en = Column(String(100), nullable=True, comment="英文名稱")
+    email_domain = Column(String(100), nullable=True,
+                          comment="郵件網域 (只有 depth=0 可設定，例如 ease.taipei)")
+    email_address = Column(String(255), nullable=True, comment="部門信箱 (例如: wind@ease.taipei)")
+    email_quota_mb = Column(Integer, default=5120, nullable=False, comment="部門信箱配額 (MB)")
+    depth = Column(Integer, default=0, nullable=False, comment="層次深度 (0=第一層，1=第二層，以此類推)")
+    description = Column(Text, comment="說明")
+
+    # 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="departments")
+    parent = relationship("Department", back_populates="children", remote_side="Department.id")
+    children = relationship("Department", back_populates="parent", cascade="all, delete-orphan")
+    members = relationship(
+        "DepartmentMember",
+        back_populates="department",
+        cascade="all, delete-orphan",
+        lazy="dynamic"
+    )
+
+    def __repr__(self):
+        return f"<Department depth={self.depth} code={self.code} name={self.name}>"
+
+    @property
+    def effective_email_domain(self) -> str | None:
+        """有效郵件網域 (第一層自身設定，子層追溯上層)"""
+        if self.depth == 0:
+            return self.email_domain
+        if self.parent:
+            return self.parent.effective_email_domain
+        return None
+
+    @property
+    def is_top_level(self) -> bool:
+        """是否為第一層部門 (原事業部)"""
+        return self.depth == 0 and self.parent_id is None