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/db/session_old.py

@@ -0,0 +1,52 @@
+"""
+資料庫連線管理
+"""
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, Session
+from typing import Generator
+
+from app.core.config import settings
+
+# 延遲初始化避免模組導入時就連接資料庫
+_engine = None
+_SessionLocal = None
+
+def get_engine():
+    """獲取資料庫引擎 (延遲初始化)"""
+    global _engine
+    if _engine is None:
+        _engine = create_engine(
+            settings.DATABASE_URL,
+            echo=settings.DATABASE_ECHO,
+            pool_pre_ping=True,
+            pool_size=10,
+            max_overflow=20,
+        )
+    return _engine
+
+def get_session_local():
+    """獲取 Session 工廠 (延遲初始化)"""
+    global _SessionLocal
+    if _SessionLocal is None:
+        _SessionLocal = sessionmaker(
+            autocommit=False,
+            autoflush=False,
+            bind=get_engine(),
+        )
+    return _SessionLocal
+
+# 向後兼容
+engine = property(lambda self: get_engine())
+SessionLocal = property(lambda self: get_session_local())
+
+
+def get_db() -> Generator[Session, None, None]:
+    """
+    取得資料庫 Session
+    用於 FastAPI 依賴注入
+    """
+    db = get_session_local()()
+    try:
+        yield db
+    finally:
+        db.close()