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/server/server.go

@@ -185,7 +185,12 @@ func Start(cfg ServerConfig) {
 			http.Error(w, "Unauthorized: UPKEEP_CLUSTER_SECRET required", 401)
 			return
 		}
-		data := store.Get().ExportData()
+		data, err := store.Get().ExportData()
+		if err != nil {
+			log.Printf("Export failed: %v", err)
+			http.Error(w, "Export failed", 500)
+			return
+		}
 		json.NewEncoder(w).Encode(data)
 	})