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/tui/tab_alerts.go

@@ -2,6 +2,7 @@ package tui
 
 import (
 	"fmt"
+	"go-upkeep/internal/monitor"
 	"go-upkeep/internal/store"
 
 	tea "github.com/charmbracelet/bubbletea"
@@ -277,9 +278,13 @@ func (m *Model) submitAlertForm() {
 	}
 
 	if m.editID > 0 {
-		store.Get().UpdateAlert(m.editID, d.Name, d.AlertType, settings)
+		if err := store.Get().UpdateAlert(m.editID, d.Name, d.AlertType, settings); err != nil {
+			monitor.AddLog("Update alert failed: " + err.Error())
+		}
 	} else {
-		store.Get().AddAlert(d.Name, d.AlertType, settings)
+		if err := store.Get().AddAlert(d.Name, d.AlertType, settings); err != nil {
+			monitor.AddLog("Add alert failed: " + err.Error())
+		}
 	}
 	m.state = stateDashboard
 }