fix(metrics): 實際呼叫 record_alert_chain_success (Wave A.5)

alert_chain_last_success_timestamp 指標已定義但從未被 set。
在 alertmanager_webhook 兩個主要成功路徑呼叫 record_alert_chain_success():
- CI/CD 告警成功處理後
- LLM 分析完成後

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

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

@@ -32,6 +32,7 @@ from pydantic import BaseModel, Field
 from src.core.config import settings
 from src.core.constants import is_cicd_alertname
 from src.core.logging import get_logger
+from src.core.metrics import record_alert_chain_success
 
 # Phase 15.2: Trace Context (moved to SignalProducerService)
 # get_trace_context 已移至 Service 層
@@ -937,6 +938,7 @@ async def alertmanager_webhook(
                 workflow_url=workflow_url,
             )
 
+            record_alert_chain_success("alertmanager")
             return AlertResponse(
                 success=True,
                 message=f"CI/CD alert processed (simple format): {alertname}",
@@ -1149,6 +1151,7 @@ async def alertmanager_webhook(
                 ai_provider=ai_provider,
             )
 
+            record_alert_chain_success("alertmanager")
             return AlertResponse(
                 success=True,
                 message=f"✅ LLM 分析完成 (via {ai_provider})",