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/tui.go

@@ -175,6 +175,7 @@ type Model struct {
 	deleteName string
 	deleteTab  int
 
+	ctx        context.Context
 	collapsed  map[int]bool
 	store      store.Store
 	engine     *monitor.Engine
@@ -213,14 +214,14 @@ type Model struct {
 	version  string
 }
 
-func InitialModel(isAdmin bool, s store.Store, eng *monitor.Engine, version string) Model {
+func InitialModel(ctx context.Context, isAdmin bool, s store.Store, eng *monitor.Engine, version string) Model {
 	vpLogs := viewport.New(100, 20)
 	vpLogs.SetContent("Waiting for logs...")
 	z := zone.New()
 	spring := harmonica.NewSpring(harmonica.FPS(10), 6.0, 0.4)
-	collapsed := loadCollapsed(s)
+	collapsed := loadCollapsed(ctx, s)
 
-	themeName, _ := s.GetPreference(context.Background(), "theme")
+	themeName, _ := s.GetPreference(ctx, "theme")
 	theme := themeByName(themeName)
 	themeIdx := 0
 	for i, t := range themes {
@@ -230,9 +231,10 @@ func InitialModel(isAdmin bool, s store.Store, eng *monitor.Engine, version stri
 		}
 	}
 
-	detailPref, _ := s.GetPreference(context.Background(), "detail_open")
+	detailPref, _ := s.GetPreference(ctx, "detail_open")
 
 	return Model{
+		ctx:             ctx,
 		state:           stateDashboard,
 		logViewport:     vpLogs,
 		maxTableRows:    5,