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>

frontend/services/tenant.service.ts

@@ -0,0 +1,60 @@
+import axios from 'axios'
+import type {
+  Tenant,
+  TenantUpdateRequest,
+  TenantUpdateResponse,
+  TenantListResponse,
+  TenantCreateRequest,
+  TenantCreateResponse,
+} from '../types/tenant'
+
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:10181'
+
+class TenantService {
+  /**
+   * Get all tenants
+   */
+  async getTenants(): Promise<TenantListResponse> {
+    const response = await axios.get<TenantListResponse>(
+      `${API_BASE_URL}/api/v1/tenants`
+    )
+    return response.data
+  }
+
+  /**
+   * Get tenant information by ID
+   */
+  async getTenant(tenantId: number): Promise<Tenant> {
+    const response = await axios.get<Tenant>(
+      `${API_BASE_URL}/api/v1/tenants/${tenantId}`
+    )
+    return response.data
+  }
+
+  /**
+   * Create new tenant
+   */
+  async createTenant(data: TenantCreateRequest): Promise<TenantCreateResponse> {
+    const response = await axios.post<TenantCreateResponse>(
+      `${API_BASE_URL}/api/v1/tenants`,
+      data
+    )
+    return response.data
+  }
+
+  /**
+   * Update tenant information
+   */
+  async updateTenant(
+    tenantId: number,
+    data: TenantUpdateRequest
+  ): Promise<TenantUpdateResponse> {
+    const response = await axios.put<TenantUpdateResponse>(
+      `${API_BASE_URL}/api/v1/tenants/${tenantId}`,
+      data
+    )
+    return response.data
+  }
+}
+
+export const tenantService = new TenantService()