refactor: propagate context.Context through call chains

Thread context.Context from callers instead of creating
context.Background() at every call site. Engine stores its
lifecycle ctx for use by triggerAlert goroutines. TUI Model
carries ctx for store operations in Cmd closures. CLI commands
create a root ctx and thread it through openStore, seed
functions, and init methods.

Only two intentional context.Background() remain in non-root
positions: engine constructor default (overridden by Start)
and drainWrites (parent already cancelled at shutdown).

internal/tui/data.go

@@ -13,9 +13,9 @@ import (
 	tea "github.com/charmbracelet/bubbletea"
 )
 
-func loadCollapsed(s store.Store) map[int]bool {
+func loadCollapsed(ctx context.Context, s store.Store) map[int]bool {
 	m := make(map[int]bool)
-	raw, err := s.GetPreference(context.Background(), "collapsed_groups")
+	raw, err := s.GetPreference(ctx, "collapsed_groups")
 	if err != nil || raw == "" {
 		return m
 	}
@@ -163,9 +163,9 @@ func (m *Model) loadTabDataCmd() tea.Cmd {
 	m.tabSeq++
 	seq := m.tabSeq
 	st := m.store
+	ctx := m.ctx
 	isAdmin := m.isAdmin
 	return func() tea.Msg {
-		ctx := context.Background()
 		alerts, err := st.GetAllAlerts(ctx)
 		if err != nil {
 			return tabDataMsg{seq: seq, err: err}
@@ -192,6 +192,7 @@ func (m *Model) loadTabDataCmd() tea.Cmd {
 // goroutine. View renders the cached result rather than querying the DB.
 func (m *Model) loadDetailCmd(siteID int) tea.Cmd {
 	eng := m.engine
+	ctx := m.ctx
 	var currentStatus models.Status
 	for _, s := range m.sites {
 		if s.ID == siteID {
@@ -200,9 +201,9 @@ func (m *Model) loadDetailCmd(siteID int) tea.Cmd {
 		}
 	}
 	return func() tea.Msg {
-		changes := eng.GetStateChanges(siteID, 5)
+		changes := eng.GetStateChanges(ctx, siteID, 5)
 		now := time.Now()
-		allChanges := eng.GetStateChangesSince(siteID, now.Add(-stateHistoryLookback))
+		allChanges := eng.GetStateChangesSince(ctx, siteID, now.Add(-stateHistoryLookback))
 		daily := monitor.ComputeDailyBreakdown(allChanges, currentStatus, stateHistoryDays, now)
 		return detailDataMsg{siteID: siteID, changes: changes, dailyDays: daily}
 	}
@@ -212,8 +213,9 @@ func (m *Model) loadDetailCmd(siteID int) tea.Cmd {
 // the UI goroutine.
 func (m *Model) loadHistoryCmd(siteID int) tea.Cmd {
 	eng := m.engine
+	ctx := m.ctx
 	return func() tea.Msg {
-		return historyDataMsg{siteID: siteID, changes: eng.GetStateChanges(siteID, stateHistoryLimit)}
+		return historyDataMsg{siteID: siteID, changes: eng.GetStateChanges(ctx, siteID, stateHistoryLimit)}
 	}
 }
 
@@ -222,12 +224,13 @@ func (m *Model) loadHistoryCmd(siteID int) tea.Cmd {
 // can be recognized and dropped.
 func (m *Model) loadSLACmd(siteID, periodIdx int) tea.Cmd {
 	eng := m.engine
+	ctx := m.ctx
 	since := time.Now().Add(-slaPeriods[periodIdx].duration)
 	return func() tea.Msg {
 		return slaDataMsg{
 			siteID:    siteID,
 			periodIdx: periodIdx,
-			changes:   eng.GetStateChangesSince(siteID, since),
+			changes:   eng.GetStateChangesSince(ctx, siteID, since),
 		}
 	}
 }