fix(kb): Signal 無 description 欄位，改用 alert_name + annotations

knowledge_extractor_service 兩處直接訪問 s.description：
- L87 signals_text 組裝：改用 alert_name + annotations.summary/description
- L198 Fallback 標題：改用 alert_name[:60]

Signal model 只有 alert_name, annotations(dict)，無 description 屬性。
此修復防止 KB 萃取時 AttributeError 導致草稿無法建立。

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

apps/api/src/services/knowledge_extractor_service.py

@@ -83,8 +83,10 @@ class KnowledgeExtractorService:
         """
         try:
             # 1. 組 Prompt
+            # 2026-04-16 ogt: Signal 無 description 欄位，用 alert_name + annotations.summary
             signals_text = "\n".join(
-                f"- {s.description}" for s in (incident.signals or [])
+                f"- {s.alert_name}: {s.annotations.get('summary', s.annotations.get('description', ''))}"
+                for s in (incident.signals or [])
             ) or "（無信號記錄）"
 
             prompt = _PROMPT_TEMPLATE.format(
@@ -194,8 +196,9 @@ class KnowledgeExtractorService:
                     return title[:200]  # DB column max 255
 
         # Fallback
+        # 2026-04-16 ogt: Signal 無 description 欄位，改用 alert_name
         signals = incident.signals or []
-        desc = signals[0].description[:60] if signals else "未知事件"
+        desc = signals[0].alert_name[:60] if signals else "未知事件"
         return f"[AI 萃取] {incident.incident_id}: {desc}"
 
     def _infer_category(self, incident) -> str: