feat(incident): Phase 27 frequency_snapshot DB 持久化 — incidents 表新增 JSONB 欄位

frequency_stats 原僅存 Redis(TTL 35天)，Pod 重啟或超期即失
- incidents.frequency_snapshot JSONB：建立 incident 時寫入快照，永久保存
- incident_repository: _record_to_incident 還原 IncidentFrequencyStats
- _incident_to_record_data 序列化 frequency_stats 快照到 DB
- Migration: phase27_incident_frequency_snapshot.sql 已執行完成

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

apps/api/src/db/models.py

@@ -565,6 +565,15 @@ class IncidentRecord(Base):
         comment="事件結果與人類回饋",
     )
 
+    # === 頻率快照 (Phase 27, 2026-04-10 ogt) ===
+    # frequency_stats 原本只存記憶體/Redis(TTL=35天)，Pod重啟或超期即失
+    # 此欄位在 incident 建立時寫入快照，永久保存當時的頻率統計
+    frequency_snapshot: Mapped[dict[str, Any] | None] = mapped_column(
+        JSON,
+        nullable=True,
+        comment="建立時刻的 AnomalyFrequency 快照，永久保存 (Phase 27)",
+    )
+
     # === 時間軸 ===
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),

apps/api/src/repositories/incident_repository.py

@@ -19,7 +19,7 @@ from sqlalchemy import select
 
 from src.db.base import get_db_context
 from src.db.models import IncidentRecord
-from src.models.incident import Incident, IncidentStatus, Severity
+from src.models.incident import Incident, IncidentFrequencyStats, IncidentStatus, Severity
 from src.repositories.interfaces import IIncidentRepository
 
 logger = structlog.get_logger(__name__)
@@ -31,6 +31,14 @@ logger = structlog.get_logger(__name__)
 
 def _record_to_incident(record: IncidentRecord) -> Incident:
     """Convert DB IncidentRecord to Pydantic Incident"""
+    # Phase 27: 從 DB 快照還原 frequency_stats
+    frequency_stats = None
+    if getattr(record, "frequency_snapshot", None):
+        try:
+            frequency_stats = IncidentFrequencyStats(**record.frequency_snapshot)
+        except Exception:
+            pass  # 欄位結構不符時跳過，不影響主流程
+
     return Incident(
         incident_id=record.incident_id,
         status=IncidentStatus(record.status),
@@ -42,11 +50,17 @@ def _record_to_incident(record: IncidentRecord) -> Incident:
         updated_at=record.updated_at,
         resolved_at=record.resolved_at,
         closed_at=record.closed_at,
+        frequency_stats=frequency_stats,
     )
 
 
 def _incident_to_record_data(incident: Incident) -> dict[str, Any]:
     """Convert Pydantic Incident to dict for DB record"""
+    # Phase 27: 序列化 frequency_stats 快照到 DB
+    frequency_snapshot = None
+    if incident.frequency_stats:
+        frequency_snapshot = incident.frequency_stats.model_dump()
+
     return {
         "incident_id": incident.incident_id,
         "status": incident.status.value,
@@ -58,6 +72,7 @@ def _incident_to_record_data(incident: Incident) -> dict[str, Any]:
         "updated_at": incident.updated_at,
         "resolved_at": incident.resolved_at,
         "closed_at": incident.closed_at,
+        "frequency_snapshot": frequency_snapshot,
     }