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/user_email_alias.py

@@ -0,0 +1,51 @@
+"""
+用戶郵件別名 Model
+支援員工在不同網域擁有多個郵件地址
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, ForeignKey, UniqueConstraint, Index
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class UserEmailAlias(Base):
+    """用戶郵件別名表 (一個用戶可以有多個郵件地址)"""
+
+    __tablename__ = "user_email_aliases"
+    __table_args__ = (
+        # 每個用戶只能有一個主要郵件
+        Index("idx_user_primary_email", "user_id", unique=True, postgresql_where=Column("is_primary") == True),
+        # 一般索引
+        Index("idx_email_aliases_user", "user_id"),
+        Index("idx_email_aliases_tenant", "tenant_id"),
+        Index("idx_email_aliases_domain", "domain_id"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("employees.id", ondelete="CASCADE"), nullable=False, index=True)
+    tenant_id = Column(Integer, ForeignKey("organizes.id", ondelete="CASCADE"), nullable=False, index=True)
+    domain_id = Column(Integer, ForeignKey("tenant_domains.id", ondelete="CASCADE"), nullable=False, index=True)
+
+    email = Column(String(150), unique=True, nullable=False, index=True, comment="郵件地址 (sales@brand-a.com)")
+    is_primary = Column(Boolean, default=False, nullable=False, comment="是否為主要郵件地址")
+
+    # 時間記錄
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+
+    # 關聯
+    user = relationship("Employee", back_populates="email_aliases")
+    domain = relationship("TenantDomain", back_populates="email_aliases")
+
+    def __repr__(self):
+        return f"<UserEmailAlias {self.email} ({'主要' if self.is_primary else '別名'})>"
+
+    @property
+    def local_part(self) -> str:
+        """郵件前綴 (@ 之前的部分)"""
+        return self.email.split('@')[0] if '@' in self.email else self.email
+
+    @property
+    def domain_part(self) -> str:
+        """網域部分 (@ 之後的部分)"""
+        return self.email.split('@')[1] if '@' in self.email else ""