feat(backend): Phase 1-4 全新開發完成，37/37 TDD 通過

[Phase 0 Reset]
- 清除舊版 app/、alembic/versions/、雜亂測試腳本
- 新 requirements.txt (移除 caldav/redis/keycloak-lib，加入 apscheduler/croniter/docker/paramiko/ping3/dnspython)

[Phase 1 資料庫]
- 9 張資料表 SQLAlchemy Models：tenants / accounts / schedules / schedule_logs /
  tenant_schedule_results / account_schedule_results / servers / server_status_logs / system_status_logs
- Alembic migration 001_create_all_tables (已套用到 10.1.0.20:5433/virtual_mis)
- seed.py：schedules 初始 3 筆 / servers 初始 4 筆

[Phase 2 CRUD API]
- GET/POST/PUT/DELETE: /api/v1/tenants / accounts / servers / schedules
- /api/v1/system-status
- 帳號編碼自動產生 (prefix + seq_no 4碼左補0)
- 燈號 (lights) 從最新排程結果取得

[Phase 3 Watchdog]
- APScheduler interval 3分鐘，原子 UPDATE status=Going 防重複執行
- 手動觸發 API: POST /api/v1/schedules/{id}/run

[Phase 4 Service Clients]
- KeycloakClient：vmis-admin realm，REST API (不用 python-keycloak)
- MailClient：Docker Mailserver @ 10.1.0.254:8080，含 MX DNS 驗證
- DockerClient：docker-py 本機 + paramiko SSH 遠端 compose
- NextcloudClient：OCS API user/quota
- SystemChecker：功能驗證 (traefik routers>0 / keycloak token / SMTP EHLO / DB SELECT 1 / ping)

[TDD]
- 37 tests / 37 passed (2.11s)
- SQLite in-memory + StaticPool，無需外部 DB

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -0,0 +1,87 @@
+"""
+pytest fixtures for VMIS backend testing.
+Uses SQLite in-memory DB (no external DB required for unit tests).
+"""
+import pytest
+from unittest.mock import patch
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import StaticPool
+
+from app.core.database import Base, get_db
+
+# ⚠️ Import all models BEFORE create_all so SQLAlchemy registers the tables
+import app.models  # noqa: F401
+
+from app.main import app
+
+TEST_DATABASE_URL = "sqlite:///:memory:"
+
+# StaticPool ensures all connections share the same in-memory SQLite database
+engine = create_engine(
+    TEST_DATABASE_URL,
+    connect_args={"check_same_thread": False},
+    poolclass=StaticPool,
+)
+TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+
+@pytest.fixture(scope="function")
+def db():
+    """Create fresh DB tables for each test."""
+    Base.metadata.create_all(bind=engine)
+    session = TestingSessionLocal()
+    try:
+        yield session
+    finally:
+        session.close()
+        Base.metadata.drop_all(bind=engine)
+
+
+@pytest.fixture(scope="function")
+def client(db):
+    """TestClient with DB override. Patches lifespan to skip real DB/scheduler."""
+    def override_get_db():
+        try:
+            yield db
+        finally:
+            pass
+
+    app.dependency_overrides[get_db] = override_get_db
+    # Patch lifespan startup/shutdown to avoid connecting to real DB or starting scheduler
+    with patch("app.main.seed_initial_data"), \
+         patch("app.main.start_watchdog"), \
+         patch("app.main.stop_watchdog"):
+        with TestClient(app, raise_server_exceptions=True) as c:
+            yield c
+    app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def sample_tenant(client):
+    """Create and return a sample tenant."""
+    payload = {
+        "code": "TEST01",
+        "prefix": "TC",
+        "name": "測試租戶一",
+        "domain": "test01.lab.taipei",
+        "status": "trial",
+    }
+    resp = client.post("/api/v1/tenants", json=payload)
+    assert resp.status_code == 201
+    return resp.json()
+
+
+@pytest.fixture
+def sample_account(client, sample_tenant):
+    """Create and return a sample account."""
+    payload = {
+        "tenant_id": sample_tenant["id"],
+        "sso_account": "user01",
+        "notification_email": "user01@external.com",
+        "quota_limit": 20,
+    }
+    resp = client.post("/api/v1/accounts", json=payload)
+    assert resp.status_code == 201
+    return resp.json()