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/permission.py

@@ -0,0 +1,112 @@
+"""
+系統權限 Model
+管理員工在各系統的存取權限 (Gitea, Portainer, etc.)
+符合設計文件規範: HR Portal設計文件.md
+"""
+from datetime import datetime
+from sqlalchemy import Column, Integer, String, DateTime, ForeignKey, Index, UniqueConstraint
+from sqlalchemy.orm import relationship
+
+from app.db.base import Base
+
+
+class Permission(Base):
+    """系統權限表"""
+
+    __tablename__ = "tenant_permissions"
+    __table_args__ = (
+        # 同一員工在同一系統只能有一個權限記錄
+        UniqueConstraint("employee_id", "system_name", name="uq_employee_system"),
+        # 索引
+        Index("idx_permissions_employee", "employee_id"),
+        Index("idx_permissions_tenant", "tenant_id"),
+        Index("idx_permissions_system", "system_name"),
+    )
+
+    id = Column(Integer, primary_key=True, index=True)
+    tenant_id = Column(
+        Integer,
+        ForeignKey("tenants.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="租戶 ID"
+    )
+    employee_id = Column(
+        Integer,
+        ForeignKey("tenant_employees.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="員工 ID"
+    )
+
+    # 權限設定
+    system_name = Column(
+        String(100),
+        nullable=False,
+        index=True,
+        comment="系統名稱 (gitea, portainer, traefik, keycloak)"
+    )
+    access_level = Column(
+        String(50),
+        default='user',
+        nullable=False,
+        comment="存取層級 (admin/user/readonly)"
+    )
+
+    # 授予資訊
+    granted_at = Column(
+        DateTime,
+        default=datetime.utcnow,
+        nullable=False,
+        comment="授予時間"
+    )
+    granted_by = Column(
+        Integer,
+        ForeignKey("tenant_employees.id", ondelete="SET NULL"),
+        nullable=True,
+        comment="授予人 (員工 ID)"
+    )
+
+    # 通用欄位 (Note: Permission 表不需要 is_active，依靠 granted_at 判斷)
+    edit_by = Column(String(100), nullable=True, comment="最後編輯者")
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False, comment="建立時間")
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False, comment="更新時間")
+
+    # 關聯
+    employee = relationship(
+        "Employee",
+        foreign_keys=[employee_id],
+        back_populates="permissions"
+    )
+    granted_by_employee = relationship(
+        "Employee",
+        foreign_keys=[granted_by]
+    )
+    granter = relationship(
+        "Employee",
+        foreign_keys=[granted_by],
+        viewonly=True,
+    )
+    tenant = relationship("Tenant")
+
+    def __repr__(self):
+        return f"<Permission {self.system_name}:{self.access_level}>"
+
+    @classmethod
+    def get_available_systems(cls) -> list[str]:
+        """取得可用的系統清單"""
+        return [
+            "gitea",       # Git 代碼託管
+            "portainer",   # 容器管理
+            "traefik",     # 反向代理管理
+            "keycloak",    # SSO 管理
+        ]
+
+    @classmethod
+    def get_available_access_levels(cls) -> list[str]:
+        """取得可用的存取層級"""
+        return [
+            "admin",      # 管理員 (完整控制)
+            "user",       # 一般使用者
+            "readonly",   # 唯讀
+        ]