fix(tui,status,store): add delete confirm, input validation, XSS fix, history persistence

Prevent accidental deletes with y/n confirmation dialog. Validate all
numeric form inputs (interval, port, timeout, threshold, retries) with
range checks instead of silently defaulting to zero. Escape user-supplied
data in status page JavaScript to close XSS via monitor names. Persist
check history to new check_history table so sparklines and uptime
percentages survive restarts.

internal/store/store.go

@@ -27,7 +27,11 @@ type Store interface {
 	UpdateUser(id int, username, publicKey, role string) error
 	DeleteUser(id int) error
 
-	// Phase 5: Backup & Restore
+	// History
+	SaveCheck(siteID int, latencyNs int64, isUp bool)
+	LoadAllHistory(limit int) map[int][]models.CheckRecord
+
+	// Backup & Restore
 	ExportData() models.Backup
 	ImportData(data models.Backup) error
 }