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/usage_log.py

@@ -0,0 +1,56 @@
+"""
+使用量記錄 Model
+記錄租戶和用戶的資源使用情況
+"""
+from datetime import datetime, date
+from sqlalchemy import Column, Integer, String, Date, DateTime, Numeric, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class UsageLog(Base):
+    """使用量記錄表 (每日統計)"""
+
+    __tablename__ = "usage_logs"
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "user_id", "date", name="uq_usage_tenant_user_date"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(Integer, ForeignKey("organizes.id", ondelete="CASCADE"), nullable=False, index=True)
+    user_id = Column(Integer, ForeignKey("employees.id", ondelete="CASCADE"), nullable=True, index=True)
+    date = Column(Date, nullable=False, index=True, comment="統計日期")
+
+    # 郵件使用量
+    email_storage_gb = Column(Numeric(10, 2), default=0, nullable=False, comment="郵件儲存 (GB)")
+    emails_sent = Column(Integer, default=0, nullable=False, comment="發送郵件數")
+    emails_received = Column(Integer, default=0, nullable=False, comment="接收郵件數")
+
+    # 雲端硬碟使用量
+    drive_storage_gb = Column(Numeric(10, 2), default=0, nullable=False, comment="硬碟儲存 (GB)")
+    files_uploaded = Column(Integer, default=0, nullable=False, comment="上傳檔案數")
+    files_downloaded = Column(Integer, default=0, nullable=False, comment="下載檔案數")
+
+    # 時間記錄
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+
+    def __repr__(self):
+        return f"<UsageLog Tenant#{self.tenant_id} User#{self.user_id} {self.date}>"
+
+    @property
+    def total_storage_gb(self) -> float:
+        """總儲存使用量 (GB)"""
+        return float(self.email_storage_gb) + float(self.drive_storage_gb)
+
+    @classmethod
+    def get_or_create(cls, tenant_id: int, user_id: int = None, log_date: date = None):
+        """獲取或創建當日記錄"""
+        if log_date is None:
+            log_date = date.today()
+
+        return cls(
+            tenant_id=tenant_id,
+            user_id=user_id,
+            date=log_date
+        )