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/invoice.py

@@ -0,0 +1,107 @@
+"""
+發票記錄 Model
+管理租戶的帳單和發票
+"""
+from datetime import datetime, date
+from sqlalchemy import Column, Integer, String, Date, DateTime, Numeric, ForeignKey, Text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import relationship
+import enum
+
+from app.db.base import Base
+
+
+class InvoiceStatus(str, enum.Enum):
+    """發票狀態"""
+    PENDING = "pending"       # 待付款
+    PAID = "paid"             # 已付款
+    OVERDUE = "overdue"       # 逾期未付
+    CANCELLED = "cancelled"   # 已取消
+
+
+class Invoice(Base):
+    """發票記錄表"""
+
+    __tablename__ = "invoices"
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("organizes.id", ondelete="CASCADE"), nullable=False, index=True)
+
+    # 發票資訊
+    invoice_number = Column(String(50), unique=True, nullable=False, index=True, comment="發票號碼 (INV-2026-03-001)")
+    issue_date = Column(Date, nullable=False, comment="開立日期")
+    due_date = Column(Date, nullable=False, comment="到期日")
+
+    # 金額
+    amount = Column(Numeric(10, 2), nullable=False, comment="金額 (未稅)")
+    tax = Column(Numeric(10, 2), default=0, nullable=False, comment="稅額")
+    total = Column(Numeric(10, 2), nullable=False, comment="總計 (含稅)")
+
+    # 狀態
+    status = Column(String(20), default=InvoiceStatus.PENDING, nullable=False, comment="狀態")
+
+    # 付款資訊
+    paid_at = Column(DateTime, nullable=True, comment="付款時間")
+    payment_method = Column(String(20), nullable=True, comment="付款方式 (credit_card/wire_transfer)")
+
+    # 發票明細 (JSON 格式)
+    line_items = Column(JSONB, nullable=True, comment="發票明細")
+    # 範例:
+    # [
+    #   {"description": "標準方案 (20 人)", "quantity": 1, "unit_price": 10000, "amount": 10000},
+    #   {"description": "超額用戶 (2 人)", "quantity": 2, "unit_price": 500, "amount": 1000}
+    # ]
+
+    # PDF 檔案
+    pdf_path = Column(String(200), nullable=True, comment="發票 PDF 路徑")
+
+    # 備註
+    notes = Column(Text, nullable=True, comment="備註")
+
+    # 時間記錄
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    # 關聯
+    tenant = relationship("Tenant")
+    payments = relationship(
+        "Payment",
+        back_populates="invoice",
+        cascade="all, delete-orphan",
+        lazy="selectin"
+    )
+
+    def __repr__(self):
+        return f"<Invoice {self.invoice_number} - NT$ {self.total} ({self.status})>"
+
+    @property
+    def is_paid(self) -> bool:
+        """是否已付款"""
+        return self.status == InvoiceStatus.PAID
+
+    @property
+    def is_overdue(self) -> bool:
+        """是否逾期"""
+        return (
+            self.status in [InvoiceStatus.PENDING, InvoiceStatus.OVERDUE] and
+            date.today() > self.due_date
+        )
+
+    @property
+    def days_overdue(self) -> int:
+        """逾期天數"""
+        if not self.is_overdue:
+            return 0
+        return (date.today() - self.due_date).days
+
+    def mark_as_paid(self, payment_method: str = None):
+        """標記為已付款"""
+        self.status = InvoiceStatus.PAID
+        self.paid_at = datetime.utcnow()
+        if payment_method:
+            self.payment_method = payment_method
+
+    @classmethod
+    def generate_invoice_number(cls, year: int, month: int, sequence: int) -> str:
+        """生成發票號碼"""
+        return f"INV-{year:04d}-{month:02d}-{sequence:03d}"