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/models/tenant.py

@@ -0,0 +1,37 @@
+from datetime import datetime
+from sqlalchemy import Boolean, Column, Integer, String, Text, DateTime, Date
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Tenant(Base):
+    __tablename__ = "tenants"
+
+    id = Column(Integer, primary_key=True, index=True, autoincrement=True)
+    code = Column(String(50), unique=True, nullable=False, index=True)
+    prefix = Column(String(20), nullable=False, default="")
+    name = Column(String(200), nullable=False)
+    name_eng = Column(String(200))
+    tax_id = Column(String(20))
+    domain = Column(String(200), unique=True, nullable=False, index=True)
+    address = Column(String(500))
+    tel = Column(String(50))
+    contact = Column(String(100))
+    contact_mobile = Column(String(50))
+    contact_email = Column(String(200))
+    keycloak_realm = Column(String(100))
+    plan_code = Column(String(50))
+    employee_limit = Column(Integer)
+    trial_start_date = Column(Date)
+    trial_end_date = Column(Date)
+    quota_per_user = Column(Integer, nullable=False, default=20)   # GB
+    total_quota = Column(Integer, nullable=False, default=200)     # GB
+    is_manager = Column(Boolean, nullable=False, default=False)
+    is_active = Column(Boolean, nullable=False, default=True)
+    status = Column(String(20), nullable=False, default="trial")   # trial / active / inactive
+    note = Column(Text)
+    created_at = Column(DateTime, nullable=False, default=datetime.utcnow)
+    updated_at = Column(DateTime, nullable=False, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+    accounts = relationship("Account", back_populates="tenant", cascade="all, delete-orphan")
+    schedule_results = relationship("TenantScheduleResult", back_populates="tenant")