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/tenant.py

@@ -0,0 +1,114 @@
+"""
+租戶 Model
+多租戶 SaaS 的核心 - 每個客戶公司對應一個租戶
+"""
+from datetime import datetime, date
+from sqlalchemy import Column, Integer, String, Boolean, Date, DateTime, Text
+from sqlalchemy.orm import relationship
+import enum
+
+from app.db.base import Base
+
+
+class TenantStatus(str, enum.Enum):
+    """租戶狀態"""
+    TRIAL = "trial"           # 試用中
+    ACTIVE = "active"         # 正常使用
+    SUSPENDED = "suspended"   # 暫停 (逾期未付款)
+    DELETED = "deleted"       # 已刪除
+
+
+class Tenant(Base):
+    """租戶表 (客戶組織)"""
+
+    __tablename__ = "tenants"
+
+    # 基本欄位
+    id = Column(Integer, primary_key=True, index=True)
+    code = Column(String(50), unique=True, nullable=False, index=True, comment="租戶代碼 (英文，例如 porscheworld)")
+    name = Column(String(200), nullable=False, comment="公司名稱")
+    name_eng = Column(String(200), nullable=True, comment="公司英文名稱")
+
+    # SSO 整合
+    keycloak_realm = Column(String(100), unique=True, nullable=True, index=True,
+                            comment="Keycloak Realm 名稱 (等同 code，每個組織一個獨立 Realm)")
+
+    # 公司資訊
+    tax_id = Column(String(20), nullable=True, comment="統一編號")
+    prefix = Column(String(10), nullable=False, default="ORG", comment="員工編號前綴 (例如 PWD → PWD0001)")
+    domain = Column(String(100), nullable=True, comment="主網域 (例如 porscheworld.tw)")
+    domain_set = Column(Text, nullable=True, comment="網域集合 (JSON Array，例如 [\"ease.taipei\", \"lab.taipei\"])")
+    tel = Column(String(50), nullable=True, comment="公司電話")
+    add = Column(String(500), nullable=True, comment="公司地址")
+    url = Column(String(200), nullable=True, comment="公司網站")
+
+    # 訂閱與方案
+    plan_id = Column(String(50), nullable=False, default="starter", comment="方案 ID (starter/standard/enterprise)")
+    max_users = Column(Integer, nullable=False, default=5, comment="最大用戶數")
+    storage_quota_gb = Column(Integer, nullable=False, default=100, comment="總儲存配額 (GB)")
+
+    # 狀態管理
+    status = Column(String(20), default=TenantStatus.TRIAL, nullable=False, comment="狀態")
+    is_sysmana = Column(Boolean, default=False, nullable=False, comment="是否為系統管理公司 (管理其他租戶)")
+    is_active = Column(Boolean, default=True, nullable=False, comment="是否啟用")
+
+    # 初始化狀態
+    is_initialized = Column(Boolean, default=False, nullable=False, comment="是否已完成初始化設定")
+    initialized_at = Column(DateTime, nullable=True, comment="初始化完成時間")
+    initialized_by = Column(String(255), nullable=True, comment="執行初始化的使用者名稱")
+
+    # 時間記錄（通用欄位）
+    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
+    departments = relationship(
+        "Department",
+        back_populates="tenant",
+        cascade="all, delete-orphan",
+        lazy="dynamic"
+    )
+    employees = relationship(
+        "Employee",
+        back_populates="tenant",
+        cascade="all, delete-orphan",
+        lazy="dynamic"
+    )
+    user_roles = relationship(
+        "UserRole",
+        back_populates="tenant",
+        cascade="all, delete-orphan",
+        lazy="dynamic"
+    )
+
+    def __repr__(self):
+        return f"<Tenant {self.code} - {self.name}>"
+
+    # is_active 已改為資料庫欄位，移除 @property
+
+    @property
+    def is_trial(self) -> bool:
+        """是否為試用狀態"""
+        return self.status == TenantStatus.TRIAL
+
+    @property
+    def total_users(self) -> int:
+        """總用戶數"""
+        return self.employees.count()
+
+    @property
+    def is_over_user_limit(self) -> bool:
+        """是否超過用戶數限制"""
+        return self.total_users > self.max_users
+
+    @property
+    def domains(self):
+        """網域列表（從 domain_set JSON 解析）"""
+        if not self.domain_set:
+            return []
+        import json
+        try:
+            return json.loads(self.domain_set)
+        except:
+            return []