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/scheduler/runner.py

@@ -0,0 +1,59 @@
+"""
+Schedule dispatcher: routes schedule_id to the correct run function.
+Also used by manual trigger API.
+"""
+import logging
+from sqlalchemy.orm import Session
+
+from app.core.database import SessionLocal
+
+logger = logging.getLogger(__name__)
+
+
+def dispatch_schedule(schedule_id: int, log_id: int = None, db: Session = None):
+    """
+    Dispatch to correct schedule function.
+    When called from watchdog, db and log_id are provided.
+    When called from manual API, creates its own session and log.
+    """
+    own_db = db is None
+    if own_db:
+        db = SessionLocal()
+
+    if log_id is None:
+        from datetime import datetime
+        from app.models.schedule import ScheduleLog, Schedule
+        schedule = db.get(Schedule, schedule_id)
+        if not schedule:
+            if own_db:
+                db.close()
+            return
+        log = ScheduleLog(
+            schedule_id=schedule_id,
+            schedule_name=schedule.name,
+            started_at=datetime.utcnow(),
+            status="running",
+        )
+        db.add(log)
+        db.commit()
+        db.refresh(log)
+        log_id = log.id
+
+    try:
+        if schedule_id == 1:
+            from app.services.scheduler.schedule_tenant import run_tenant_check
+            run_tenant_check(log_id, db)
+        elif schedule_id == 2:
+            from app.services.scheduler.schedule_account import run_account_check
+            run_account_check(log_id, db)
+        elif schedule_id == 3:
+            from app.services.scheduler.schedule_system import run_system_status
+            run_system_status(log_id, db)
+        else:
+            logger.warning(f"Unknown schedule_id: {schedule_id}")
+    except Exception as e:
+        logger.exception(f"dispatch_schedule({schedule_id}) error: {e}")
+        raise
+    finally:
+        if own_db:
+            db.close()