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/schemas/server.py

@@ -0,0 +1,40 @@
+from datetime import datetime
+from typing import Optional
+from pydantic import BaseModel
+
+
+class ServerBase(BaseModel):
+    name: str
+    ip_address: str
+    description: Optional[str] = None
+    sort_order: int = 0
+    is_active: bool = True
+
+
+class ServerCreate(ServerBase):
+    pass
+
+
+class ServerUpdate(BaseModel):
+    name: Optional[str] = None
+    ip_address: Optional[str] = None
+    description: Optional[str] = None
+    sort_order: Optional[int] = None
+    is_active: Optional[bool] = None
+
+
+class ServerAvailability(BaseModel):
+    availability_30d: Optional[float] = None
+    availability_90d: Optional[float] = None
+    availability_365d: Optional[float] = None
+
+
+class ServerResponse(ServerBase):
+    id: int
+    recorded_at: datetime
+    last_result: Optional[bool] = None
+    last_response_time: Optional[float] = None
+    availability: Optional[ServerAvailability] = None
+
+    class Config:
+        from_attributes = True