feat(api): Add Alertmanager native format webhook endpoint

- POST /api/v1/webhooks/alertmanager accepts Prometheus Alertmanager format
- Internal IPs (192.168.x.x, 10.x.x.x) bypass HMAC verification
- Converts Alertmanager alerts to Signal format → Redis Stream
- External IPs must use /signals with HMAC

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/api/v1/webhooks.py

@@ -977,6 +977,123 @@ async def receive_alert(
         ) from e
 
 
+# =============================================================================
+# Phase 10: Alertmanager 原生格式支援 (內網免 HMAC)
+# =============================================================================
+
+class AlertmanagerAlert(BaseModel):
+    """Alertmanager 單一告警"""
+    status: str  # firing, resolved
+    labels: dict = {}
+    annotations: dict = {}
+    startsAt: str | None = None
+    endsAt: str | None = None
+    generatorURL: str | None = None
+    fingerprint: str | None = None
+
+
+class AlertmanagerPayload(BaseModel):
+    """Alertmanager Webhook Payload"""
+    version: str | None = "4"
+    groupKey: str | None = None
+    status: str  # firing, resolved
+    receiver: str | None = None
+    groupLabels: dict | None = {}
+    commonLabels: dict | None = {}
+    commonAnnotations: dict | None = {}
+    externalURL: str | None = None
+    alerts: list[AlertmanagerAlert]
+
+
+def is_internal_ip(client_ip: str) -> bool:
+    """檢查是否為內網 IP"""
+    import ipaddress
+    try:
+        ip = ipaddress.ip_address(client_ip)
+        # 私有網段: 10.x.x.x, 172.16-31.x.x, 192.168.x.x, localhost
+        return ip.is_private or ip.is_loopback
+    except ValueError:
+        return False
+
+
+@router.post(
+    "/alertmanager",
+    response_model=SignalResponse,
+    summary="Phase 10: Alertmanager 原生格式 (內網免 HMAC)",
+    description="接收 Alertmanager Webhook，內網來源免 HMAC 驗證。",
+)
+async def alertmanager_webhook(
+    request: Request,
+    payload: AlertmanagerPayload,
+) -> SignalResponse:
+    """
+    接收 Alertmanager 格式告警並轉換為 Signal
+
+    安全策略:
+    - 內網 IP (192.168.x.x, 10.x.x.x): 免 HMAC
+    - 外網 IP: 拒絕 (需使用 /signals 端點)
+    """
+    # 取得客戶端 IP
+    client_ip = request.client.host if request.client else "unknown"
+    forwarded_for = request.headers.get("X-Forwarded-For", "").split(",")[0].strip()
+    actual_ip = forwarded_for or client_ip
+
+    # 內網檢查
+    if not is_internal_ip(actual_ip):
+        logger.warning(
+            "alertmanager_external_rejected",
+            client_ip=actual_ip,
+            reason="External IP must use /signals with HMAC",
+        )
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="External sources must use /signals endpoint with HMAC signature",
+        )
+
+    logger.info(
+        "alertmanager_webhook_received",
+        client_ip=actual_ip,
+        status=payload.status,
+        alert_count=len(payload.alerts),
+    )
+
+    # 處理每個告警
+    message_ids = []
+    for alert in payload.alerts:
+        if alert.status != "firing":
+            continue  # 只處理 firing 狀態
+
+        # 轉換為 SignalPayload
+        severity_map = {"critical": "critical", "warning": "warning", "info": "info"}
+        severity = severity_map.get(
+            alert.labels.get("severity", "warning").lower(),
+            "warning"
+        )
+
+        signal = SignalPayload(
+            source="alertmanager",
+            alert_name=alert.labels.get("alertname", "UnknownAlert"),
+            severity=severity,
+            namespace=alert.labels.get("namespace", "default"),
+            target=alert.labels.get("pod", alert.labels.get("instance", "unknown")),
+            message=alert.annotations.get("summary", alert.annotations.get("description", "")),
+            labels=alert.labels,
+            annotations=alert.annotations,
+        )
+
+        try:
+            message_id = await produce_signal_to_stream(signal)
+            message_ids.append(message_id)
+        except Exception as e:
+            logger.error("alertmanager_signal_produce_error", error=str(e))
+
+    return SignalResponse(
+        success=True,
+        message_id=message_ids[0] if message_ids else None,
+        stream=SIGNAL_STREAM_KEY,
+    )
+
+
 @router.get(
     "/health",
     summary="Webhook 健康檢查",