Event log: show only the events with a failed or a pending delivery (closes #390)

The event log gains three links above the list: All, Failed (N) and
Pending (N), carried in a `show` query parameter; any other value shows
every event. Failed lists the events with a failed delivery, Pending
those with one pending or retrying, each once, under the full list's
50-row limit and order. A filtered list finds the matching deliveries
through `idx_deliveries_status`, looks up their events by ID and sorts
them, and reads the rows of only the events shown; its counts read the
deliveries alone. The line beside the heading says what a filter
counts, and Replay returns to the list it was pressed in. The README
and the comments naming the query parameters the service reads include
`show`.

Model: opus-5-5

internal/database/event_tier_indexes_test.go

@@ -149,6 +149,62 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		Delete(&database.Event{}), "sqlite_autoindex_events_1 (id=?)")
 }
 
+// TestEventLogFiltersUseTheStatusIndex does the same for the event log's
+// Failed and Pending lists, of the newest events with a delivery in
+// given statuses, and for their counts (eventsWithStatus and
+// countEventsWithStatus in the handlers). The lists must also reach
+// the events table only by ID: from the matching deliveries, then from
+// the newest of those events.
+func TestEventLogFiltersUseTheStatusIndex(t *testing.T) {
+	t.Parallel()
+
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhookID := uuid.New().String()
+
+	db, err := mgr.GetDB(webhookID)
+	require.NoError(t, err)
+
+	dry := db.Session(&gorm.Session{DryRun: true})
+	byStatus := "idx_deliveries_status (status=? AND deleted_at=?)"
+	pending := []database.DeliveryStatus{
+		database.DeliveryStatusPending,
+		database.DeliveryStatusRetrying,
+	}
+
+	var (
+		rows  []struct{ ID string }
+		count int64
+	)
+
+	matching := dry.Model(&database.Delivery{}).
+		Distinct("event_id").Where("status IN ?", pending)
+	newest := dry.Table("(?) AS matching", matching).
+		Joins("CROSS JOIN events ON events.id = matching.event_id").
+		Where(
+			"events.webhook_id = ? AND events.deleted_at IS NULL",
+			webhookID,
+		).
+		Order("events.created_at DESC").Limit(50).
+		Select("events.id AS event_id")
+
+	// Each step of the plan is printed in braces, so these name the
+	// lookup that follows each scan.
+	byID := "{SEARCH events USING INDEX sqlite_autoindex_events_1 (id=?)}"
+
+	assertPlanUses(t, db, dry.Table("(?) AS newest", newest).
+		Joins("CROSS JOIN events ON events.id = newest.event_id").
+		Select("id").Order("created_at DESC").Limit(50).Find(&rows),
+		byStatus, "{SCAN matching} "+byID, "{SCAN newest} "+byID)
+	assertPlanUses(t, db, dry.Model(&database.Delivery{}).
+		Distinct("event_id").Where("status IN ?", pending).Count(&count),
+		byStatus)
+}
+
 // TestStatisticsQueriesUseTheirIndexes does the same for the webhook
 // page's statistics (readEventStats in the handlers): deliveries in
 // progress, each target's deliveries finished since a time, which must