feat(CORE-001D3B): surface alert history degraded state

app/models/alerts.py

@@ -8,6 +8,12 @@ from pathlib import Path
 from typing import Any
 
 
+class AlertHistoryLoadError(RuntimeError):
+    def __init__(self, history_path: Path, message: str) -> None:
+        super().__init__(message)
+        self.history_path = history_path
+
+
 @dataclass
 class AlertEvent:
     severity: str
@@ -36,6 +42,8 @@ class AlertStatus:
     critical_threshold: float
     email_alerts_enabled: bool
     history: list[AlertEvent]
+    history_unavailable: bool = False
+    history_notice: str | None = None
 
 
 class AlertHistoryRepository:
@@ -53,10 +61,12 @@ class AlertHistoryRepository:
         try:
             with self.history_path.open() as f:
                 data = json.load(f)
-        except (json.JSONDecodeError, OSError):
-            return []
+        except json.JSONDecodeError as exc:
+            raise AlertHistoryLoadError(self.history_path, f"Alert history is not valid JSON: {exc}") from exc
+        except OSError as exc:
+            raise AlertHistoryLoadError(self.history_path, f"Alert history could not be read: {exc}") from exc
         if not isinstance(data, list):
-            return []
+            raise AlertHistoryLoadError(self.history_path, "Alert history payload must be a list")
         return [AlertEvent.from_dict(item) for item in data if isinstance(item, dict)]
 
     def save(self, events: list[AlertEvent]) -> None: