fix(tui): finish moving keypress DB reads into tea.Cmds

The #101 refactor stopped at the tick path; 'h' history and the SLA
view still queried state changes synchronously in Update, freezing the
UI for up to busy_timeout on a contended DB. Both now load through
Cmds with loading placeholders.

Also closes the remaining staleness holes in the async data flow:
- tabDataMsg carries a sequence number; out-of-order replies from
  slower earlier loads are dropped instead of overwriting newer data
- history/SLA replies are dropped when the user has navigated to a
  different site or period
- the open detail panel refreshes on the tab-data cadence instead of
  loading once on entry and going stale
- initSiteHuhForm reads the m.alerts cache instead of hitting the store

internal/tui/tab_sites.go

@@ -325,14 +325,13 @@ func (m *Model) initSiteHuhForm() tea.Cmd {
 		}
 	}
 
+	// m.alerts is the tab-data cache (≤5s stale) — no store IO in Update.
 	alertOpts := []huh.Option[string]{huh.NewOption("None", "0")}
-	if alerts, err := m.store.GetAllAlerts(); err == nil {
-		for _, a := range alerts {
-			alertOpts = append(alertOpts, huh.NewOption(
-				fmt.Sprintf("%s (%s)", a.Name, a.Type),
-				strconv.Itoa(a.ID),
-			))
-		}
+	for _, a := range m.alerts {
+		alertOpts = append(alertOpts, huh.NewOption(
+			fmt.Sprintf("%s (%s)", a.Name, a.Type),
+			strconv.Itoa(a.ID),
+		))
 	}
 
 	groupOpts := []huh.Option[string]{huh.NewOption("None", "0")}