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/app/services/nextcloud_client.py

@@ -0,0 +1,85 @@
+"""
+NextcloudClient — Nextcloud OCS API
+管理 NC 使用者的查詢/建立與 quota 統計。
+"""
+import logging
+from typing import Optional
+import httpx
+
+logger = logging.getLogger(__name__)
+
+OCS_HEADERS = {"OCS-APIRequest": "true"}
+TIMEOUT = 15.0
+
+
+class NextcloudClient:
+    def __init__(self, domain: str, admin_user: str = "admin", admin_password: str = ""):
+        self._base = f"https://{domain}"
+        self._auth = (admin_user, admin_password)
+
+    def user_exists(self, username: str) -> bool:
+        try:
+            resp = httpx.get(
+                f"{self._base}/ocs/v1.php/cloud/users/{username}",
+                auth=self._auth,
+                headers=OCS_HEADERS,
+                timeout=TIMEOUT,
+            )
+            return resp.status_code == 200
+        except Exception:
+            return False
+
+    def create_user(self, username: str, password: Optional[str], quota_gb: int = 20) -> bool:
+        try:
+            resp = httpx.post(
+                f"{self._base}/ocs/v1.php/cloud/users",
+                auth=self._auth,
+                headers=OCS_HEADERS,
+                data={
+                    "userid": username,
+                    "password": password or "",
+                    "quota": f"{quota_gb}GB",
+                },
+                timeout=TIMEOUT,
+            )
+            return resp.status_code == 200
+        except Exception as e:
+            logger.error(f"NC create_user({username}) failed: {e}")
+            return False
+
+    def get_user_quota_used_gb(self, username: str) -> Optional[float]:
+        try:
+            resp = httpx.get(
+                f"{self._base}/ocs/v2.php/cloud/users/{username}",
+                auth=self._auth,
+                headers=OCS_HEADERS,
+                timeout=TIMEOUT,
+            )
+            if resp.status_code != 200:
+                return None
+            used_bytes = resp.json().get("ocs", {}).get("data", {}).get("quota", {}).get("used", 0)
+            return round(used_bytes / 1073741824, 4)
+        except Exception:
+            return None
+
+    def get_total_quota_used_gb(self) -> Optional[float]:
+        """Sum all users' quota usage"""
+        try:
+            resp = httpx.get(
+                f"{self._base}/ocs/v2.php/cloud/users",
+                auth=self._auth,
+                headers=OCS_HEADERS,
+                params={"limit": 500},
+                timeout=TIMEOUT,
+            )
+            if resp.status_code != 200:
+                return None
+            users = resp.json().get("ocs", {}).get("data", {}).get("users", [])
+            total = 0.0
+            for uid in users:
+                used = self.get_user_quota_used_gb(uid)
+                if used:
+                    total += used
+            return round(total, 4)
+        except Exception:
+            return None