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/tui.go

@@ -121,6 +121,7 @@ type Model struct {
 	historyViewport viewport.Model
 	historyChanges  []models.StateChange
 	historySiteName string
+	historySiteID   int
 
 	slaViewport       viewport.Model
 	slaReport         monitor.SLAReport
@@ -155,6 +156,7 @@ type Model struct {
 	nodes              []models.ProbeNode
 	maintenanceWindows []models.MaintenanceWindow
 	lastTabLoad        time.Time // last dispatch of loadTabDataCmd (throttle)
+	tabSeq             int       // seq of the newest issued tab-data load
 
 	// detail-panel state-change history, loaded on enter so View does no DB IO
 	detailChanges       []models.StateChange