Add system comments on close events

Manual close: "Manually closed."
Auto-resolve: "Condition resolved — alert closed automatically."
(reopen comments were already in place)

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

src/lib/db.ts

@@ -185,6 +185,12 @@ export function upsertAlerts(candidates: AlertCandidate[]): number {
     alert.closeReason = "resolved";
     alert.closedAt = nowISO;
     alert.suppressedUntil = null;
+    alert.comments.push({
+      id: store.nextCommentId++,
+      body: "Condition resolved — alert closed automatically.",
+      createdAt: nowISO,
+      author: "system",
+    });
   }
 
   save(store);
@@ -221,10 +227,17 @@ export function closeAlert(id: number): Alert | null {
     suppressedUntil = d.toISOString();
   }
 
+  const now = new Date().toISOString();
   alert.status = "closed";
   alert.closeReason = "manual";
-  alert.closedAt = new Date().toISOString();
+  alert.closedAt = now;
   alert.suppressedUntil = suppressedUntil;
+  alert.comments.push({
+    id: store.nextCommentId++,
+    body: "Manually closed.",
+    createdAt: now,
+    author: "system",
+  });
   save(store);
   return toAlert(alert);
 }