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/update.go

@@ -1,7 +1,6 @@
 package tui
 
 import (
-	"context"
 	"fmt"
 	"time"
 
@@ -76,23 +75,24 @@ func (m *Model) handleConfirmDelete(msg tea.Msg) (tea.Model, tea.Cmd) {
 		// writeDoneMsg reload converges the UI back to the DB state (and the
 		// engine poll loop re-adds a site that is still in the DB).
 		st := m.store
+		ctx := m.ctx
 		id := m.deleteID
 		var cmd tea.Cmd
 		switch m.deleteTab {
 		case tabMonitors:
-			cmd = writeCmd("Delete site", func() error { return st.DeleteSite(context.Background(), id) })
+			cmd = writeCmd("Delete site", func() error { return st.DeleteSite(ctx, id) })
 			m.engine.RemoveSite(id)
 			m.adjustCursor(len(m.sites) - 1)
 		case tabMaint:
-			cmd = writeCmd("Delete maintenance window", func() error { return st.DeleteMaintenanceWindow(context.Background(), id) })
+			cmd = writeCmd("Delete maintenance window", func() error { return st.DeleteMaintenanceWindow(ctx, id) })
 			m.adjustCursor(len(m.maintenanceWindows) - 1)
 		case tabSettings:
 			switch m.settingsSection {
 			case sectionAlerts:
-				cmd = writeCmd("Delete alert", func() error { return st.DeleteAlert(context.Background(), id) })
+				cmd = writeCmd("Delete alert", func() error { return st.DeleteAlert(ctx, id) })
 				m.adjustCursor(len(m.alerts) - 1)
 			case sectionUsers:
-				cmd = writeCmd("Delete user", func() error { return st.DeleteUser(context.Background(), id) })
+				cmd = writeCmd("Delete user", func() error { return st.DeleteUser(ctx, id) })
 				m.adjustCursor(len(m.users) - 1)
 			}
 		}
@@ -232,8 +232,9 @@ func (m *Model) handleTabData(msg tabDataMsg) (tea.Model, tea.Cmd) {
 // surfaces through the engine log (picked up by the next refreshLive).
 func (m *Model) testAlertCmd(id int, name string) tea.Cmd {
 	eng := m.engine
+	ctx := m.ctx
 	return func() tea.Msg {
-		if err := eng.TestAlert(id); err != nil {
+		if err := eng.TestAlert(ctx, id); err != nil {
 			eng.AddLog(fmt.Sprintf("Test alert failed (%s): %v", name, err))
 		}
 		return nil
@@ -635,9 +636,10 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			m.collapsed[gid] = !m.collapsed[gid]
 			payload := collapsedJSON(m.collapsed)
 			st := m.store
+			ctx := m.ctx
 			m.refreshLive()
 			return m, writeCmd("Save collapsed groups", func() error {
-				return st.SetPreference(context.Background(), "collapsed_groups", payload)
+				return st.SetPreference(ctx, "collapsed_groups", payload)
 			})
 		}
 	case "p":
@@ -645,9 +647,10 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			id := m.sites[m.cursor].ID
 			paused := m.engine.ToggleSitePause(id)
 			st := m.store
+			ctx := m.ctx
 			m.refreshLive()
 			return m, writeCmd("Update pause state", func() error {
-				return st.UpdateSitePaused(context.Background(), id, paused)
+				return st.UpdateSitePaused(ctx, id, paused)
 			})
 		}
 	case "i":
@@ -655,6 +658,7 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			m.detailOpen = !m.detailOpen
 			m.recalcLayout()
 			st := m.store
+			ctx := m.ctx
 			open := m.detailOpen
 			var cmd tea.Cmd
 			if m.detailOpen {
@@ -665,7 +669,7 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 				if open {
 					v = "true"
 				}
-				return st.SetPreference(context.Background(), "detail_open", v)
+				return st.SetPreference(ctx, "detail_open", v)
 			})
 			if cmd != nil {
 				return m, tea.Batch(cmd, saveCmd)
@@ -682,8 +686,9 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 				m.detailOpen = false
 				m.recalcLayout()
 				st := m.store
+				ctx := m.ctx
 				return m, writeCmd("Save detail preference", func() error {
-					return st.SetPreference(context.Background(), "detail_open", "false")
+					return st.SetPreference(ctx, "detail_open", "false")
 				})
 			}
 		}
@@ -712,10 +717,11 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 			isActive := !mw.StartTime.After(now) && (mw.EndTime.IsZero() || mw.EndTime.After(now))
 			if isActive {
 				st := m.store
+				ctx := m.ctx
 				id := mw.ID
 				m.refreshLive()
 				return m, writeCmd("End maintenance", func() error {
-					return st.EndMaintenanceWindow(context.Background(), id)
+					return st.EndMaintenanceWindow(ctx, id)
 				})
 			}
 		}
@@ -724,9 +730,10 @@ func (m *Model) handleDashboardKey(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.theme = themes[m.themeIndex]
 		m.st = newStyles(m.theme)
 		st := m.store
+		ctx := m.ctx
 		name := m.theme.Name
 		return m, writeCmd("Save theme", func() error {
-			return st.SetPreference(context.Background(), "theme", name)
+			return st.SetPreference(ctx, "theme", name)
 		})
 	case "d":
 		return m.handleDeleteItem()