refactor(store): add error returns to all Store interface methods

Every Store method now returns an error. Callers handle errors
gracefully — TUI logs to event log, server returns HTTP 500,
monitor engine logs and retries. All rows.Scan() errors are now
checked in sqlstore.go instead of silently appending corrupt data.

- GetSites, GetAllAlerts, GetAllUsers return ([]T, error)
- GetAlert returns (AlertConfig, error) instead of (AlertConfig, bool)
- AddSite, UpdateSite, DeleteSite, etc. all return error
- SaveCheck, LoadAllHistory, ExportData return error
- ~25 caller sites updated across tui, server, monitor, main

internal/monitor/history.go

@@ -25,7 +25,11 @@ func InitHistoryFromStore() {
 	if s == nil {
 		return
 	}
-	all := s.LoadAllHistory(maxHistoryLen)
+	all, err := s.LoadAllHistory(maxHistoryLen)
+	if err != nil {
+		AddLog("Failed to load check history: " + err.Error())
+		return
+	}
 	historyMu.Lock()
 	defer historyMu.Unlock()
 	for siteID, records := range all {
@@ -71,7 +75,7 @@ func RecordCheck(siteID int, latency time.Duration, isUp bool) {
 	}
 
 	if s := store.Get(); s != nil {
-		go s.SaveCheck(siteID, latency.Nanoseconds(), isUp)
+		go func() { _ = s.SaveCheck(siteID, latency.Nanoseconds(), isUp) }()
 	}
 }