fix(classify): backup/heartbeat severity=warning/critical 告警恢復告警卡片格式

根因：classify_alert_early() backup 規則無 severity 條件，導致
VeleroBackupFailed / HostBackupFailed (warning/critical) 被分為 TYPE-1
（純資訊無按鈕），告警卡片格式遺失。

修復：
- backup/heartbeat 關鍵字只在 severity=info/none 才命中 TYPE-1
- severity=warning/critical 的 backup 告警走正確 prefix 規則
  (Velero→kubernetes TYPE-3, HostBackup→infrastructure TYPE-3)
- Watchdog (severity=none) 由 severity 規則先命中，維持 TYPE-1
- 補強測試：25 cases，含 VeleroBackupFailed critical → kubernetes TYPE-3

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/services/incident_service.py

@@ -126,7 +126,12 @@ def classify_alert_early(alertname: str, severity: str, _labels: dict) -> tuple[
         return "config_drift", "TYPE-4D"
     if severity in ("info", "none"):
         return "info", "TYPE-1"
-    if any(kw in alertname_lower for kw in ("backup", "heartbeat")):
+    # backup/heartbeat 關鍵字只有 severity=info/none 才是純資訊
+    # severity=warning/critical（例如 VeleroBackupFailed, HostBackupFailed）→ 繼續走 prefix 規則
+    if severity in ("info", "none") and any(kw in alertname_lower for kw in ("backup", "heartbeat")):
+        return "backup", "TYPE-1"
+    # Watchdog/Heartbeat 永遠是 TYPE-1（Alertmanager 心跳）
+    if "watchdog" in alertname_lower or alertname in ("Heartbeat",):
         return "backup", "TYPE-1"
     if alertname.startswith(("Docker", "Host")):
         return "infrastructure", "TYPE-3"