Preserve alert history when reopening

When a closed alert reopens (condition still present, no active
cooldown), keep firstSeen and comments intact. The alert is the same
incident continuing — closing and reopening should not erase history.

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

src/lib/db.ts

@@ -126,14 +126,14 @@ export function upsertAlerts(candidates: AlertCandidate[]): number {
 
       if (isSuppressed) continue;
 
-      // Re-open if previously closed (manually or resolved) and not suppressed
+      // Re-open if previously closed (manually or resolved) and not suppressed.
+      // Preserve firstSeen and comments — this is the same incident continuing,
+      // not a brand new one.
       if (existing.status === "closed") {
         existing.status = "open";
         existing.closeReason = null;
         existing.closedAt = null;
         existing.suppressedUntil = null;
-        existing.firstSeen = nowISO; // treat as a new occurrence
-        existing.comments = [];
       }
 
       // Refresh content and lastSeen