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/test_tenants.py

@@ -0,0 +1,100 @@
+"""TDD: Tenant CRUD API tests"""
+
+
+def test_create_tenant(client):
+    payload = {
+        "code": "PORS01",
+        "prefix": "PC",
+        "name": "Porsche測試公司",
+        "domain": "porsche.lab.taipei",
+        "status": "trial",
+    }
+    resp = client.post("/api/v1/tenants", json=payload)
+    assert resp.status_code == 201
+    data = resp.json()
+    assert data["code"] == "PORS01"
+    assert data["prefix"] == "PC"
+    assert data["domain"] == "porsche.lab.taipei"
+    assert data["status"] == "trial"
+    assert data["is_active"] is True
+    assert data["quota_per_user"] == 20
+    assert data["total_quota"] == 200
+    assert data["id"] is not None
+
+
+def test_create_tenant_duplicate_code(client, sample_tenant):
+    payload = {
+        "code": sample_tenant["code"],
+        "prefix": "XX",
+        "name": "重複代碼",
+        "domain": "another.lab.taipei",
+        "status": "trial",
+    }
+    resp = client.post("/api/v1/tenants", json=payload)
+    assert resp.status_code == 409
+
+
+def test_create_tenant_duplicate_domain(client, sample_tenant):
+    payload = {
+        "code": "NEWCODE",
+        "prefix": "XX",
+        "name": "重複網域",
+        "domain": sample_tenant["domain"],
+        "status": "trial",
+    }
+    resp = client.post("/api/v1/tenants", json=payload)
+    assert resp.status_code == 409
+
+
+def test_list_tenants(client, sample_tenant):
+    resp = client.get("/api/v1/tenants")
+    assert resp.status_code == 200
+    tenants = resp.json()
+    assert len(tenants) == 1
+    assert tenants[0]["code"] == sample_tenant["code"]
+
+
+def test_list_tenants_filter_active(client, sample_tenant):
+    # Deactivate
+    client.put(f"/api/v1/tenants/{sample_tenant['id']}", json={"is_active": False})
+    resp = client.get("/api/v1/tenants", params={"is_active": True})
+    assert resp.status_code == 200
+    assert len(resp.json()) == 0
+
+
+def test_get_tenant(client, sample_tenant):
+    resp = client.get(f"/api/v1/tenants/{sample_tenant['id']}")
+    assert resp.status_code == 200
+    data = resp.json()
+    assert data["id"] == sample_tenant["id"]
+    assert data["code"] == sample_tenant["code"]
+    assert data["lights"] is None  # No schedule runs yet
+
+
+def test_get_tenant_not_found(client):
+    resp = client.get("/api/v1/tenants/99999")
+    assert resp.status_code == 404
+
+
+def test_update_tenant(client, sample_tenant):
+    resp = client.put(
+        f"/api/v1/tenants/{sample_tenant['id']}",
+        json={"name": "更新後的名稱", "status": "active"},
+    )
+    assert resp.status_code == 200
+    data = resp.json()
+    assert data["name"] == "更新後的名稱"
+    assert data["status"] == "active"
+
+
+def test_delete_tenant(client, sample_tenant):
+    resp = client.delete(f"/api/v1/tenants/{sample_tenant['id']}")
+    assert resp.status_code == 204
+    resp = client.get(f"/api/v1/tenants/{sample_tenant['id']}")
+    assert resp.status_code == 404
+
+
+def test_health_endpoint(client):
+    resp = client.get("/health")
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "ok"