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/business_unit.py.deprecated

@@ -0,0 +1,49 @@
+"""
+事業部 Model
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, Text, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class BusinessUnit(Base):
+    """事業部表"""
+
+    __tablename__ = "business_units"
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "code", name="uq_tenant_bu_code"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("organizes.id", ondelete="CASCADE"), nullable=False, index=True, comment="租戶 ID")
+    name = Column(String(100), nullable=False, comment="事業部名稱")
+    name_en = Column(String(100), comment="英文名稱")
+    code = Column(String(20), nullable=False, index=True, comment="事業部代碼 (BD, TD, OM, 租戶內唯一)")
+    email_domain = Column(String(100), unique=True, nullable=False, comment="郵件網域 (ease.taipei, lab.taipei, porscheworld.tw)")
+    description = Column(Text, comment="說明")
+    is_active = Column(Boolean, default=True, nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+
+    # Phase 2.2 新增欄位
+    primary_domain = Column(String(100), comment="主要網域 (與 email_domain 相同)")
+    email_address = Column(String(255), comment="事業部信箱 (例如: business@ease.taipei)")
+    email_quota_mb = Column(Integer, default=10240, nullable=False, comment="事業部信箱配額 (MB)")
+
+    # 關聯
+    tenant = relationship("Tenant", back_populates="business_units")
+    # departments relationship 已移除 (business_unit_id FK 已從 departments 表刪除於 migration 0005)
+    employee_identities = relationship(
+        "EmployeeIdentity",
+        back_populates="business_unit",
+        lazy="dynamic"
+    )
+
+    def __repr__(self):
+        return f"<BusinessUnit {self.code} - {self.name}>"
+
+    @property
+    def sso_domain(self) -> str:
+        """SSO 帳號網域"""
+        return self.email_domain