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/system_function_cache.py

@@ -0,0 +1,31 @@
+"""
+系統功能快取 Model
+從 System Admin 服務同步的系統功能定義 (只讀副本)
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, Boolean, DateTime, UniqueConstraint, Index
+
+from app.db.base import Base
+
+
+class SystemFunctionCache(Base):
+    """系統功能快取表"""
+
+    __tablename__ = "system_functions_cache"
+    __table_args__ = (
+        UniqueConstraint("function_code", name="uq_function_code"),
+        Index("idx_func_cache_service", "service_code"),
+        Index("idx_func_cache_category", "function_category"),
+    )
+
+    id = Column(Integer, primary_key=True, comment="與 System Admin 的 id 一致")
+    service_code = Column(String(50), nullable=False, comment="服務代碼: hr/erp/mail/ai")
+    function_code = Column(String(100), nullable=False, comment="功能代碼: HR_EMPLOYEE_VIEW")
+    function_name = Column(String(200), nullable=False, comment="功能名稱")
+    function_category = Column(String(50), nullable=True,
+                               comment="功能分類: query/manage/approve/report")
+    is_active = Column(Boolean, default=True, nullable=False)
+    synced_at = Column(DateTime, default=datetime.utcnow, nullable=False, comment="最後同步時間")
+
+    def __repr__(self):
+        return f"<SystemFunctionCache {self.function_code} ({self.service_code})>"