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/subscription.py

@@ -0,0 +1,77 @@
+"""
+訂閱記錄 Model
+管理租戶的訂閱狀態和歷史
+"""
+from datetime import datetime, date
+from sqlalchemy import Column, Integer, String, Date, DateTime, Boolean, ForeignKey, Numeric
+from sqlalchemy.orm import relationship
+import enum
+
+from app.db.base import Base
+
+
+class SubscriptionStatus(str, enum.Enum):
+    """訂閱狀態"""
+    ACTIVE = "active"         # 進行中
+    CANCELLED = "cancelled"   # 已取消
+    EXPIRED = "expired"       # 已過期
+
+
+class Subscription(Base):
+    """訂閱記錄表"""
+
+    __tablename__ = "subscriptions"
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("organizes.id", ondelete="CASCADE"), nullable=False, index=True)
+
+    # 方案資訊
+    plan_id = Column(String(50), nullable=False, comment="方案 ID (starter/standard/enterprise)")
+    start_date = Column(Date, nullable=False, comment="開始日期")
+    end_date = Column(Date, nullable=False, comment="結束日期")
+    status = Column(String(20), default=SubscriptionStatus.ACTIVE, nullable=False, comment="狀態")
+
+    # 自動續約
+    auto_renew = Column(Boolean, default=True, nullable=False, comment="是否自動續約")
+
+    # 時間記錄
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    cancelled_at = Column(DateTime, nullable=True, comment="取消時間")
+
+    # 關聯
+    tenant = relationship("Tenant", back_populates="subscriptions")
+
+    def __repr__(self):
+        return f"<Subscription {self.plan_id} for Tenant#{self.tenant_id} ({self.start_date} ~ {self.end_date})>"
+
+    @property
+    def is_active(self) -> bool:
+        """是否為活躍訂閱"""
+        today = date.today()
+        return (
+            self.status == SubscriptionStatus.ACTIVE and
+            self.start_date <= today <= self.end_date
+        )
+
+    @property
+    def days_remaining(self) -> int:
+        """剩餘天數"""
+        if not self.is_active:
+            return 0
+        return (self.end_date - date.today()).days
+
+    def renew(self, months: int = 1) -> "Subscription":
+        """續約 (創建新的訂閱記錄)"""
+        from dateutil.relativedelta import relativedelta
+
+        new_start = self.end_date + relativedelta(days=1)
+        new_end = new_start + relativedelta(months=months) - relativedelta(days=1)
+
+        return Subscription(
+            tenant_id=self.tenant_id,
+            plan_id=self.plan_id,
+            start_date=new_start,
+            end_date=new_end,
+            status=SubscriptionStatus.ACTIVE,
+            auto_renew=self.auto_renew
+        )