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

The event log had no way to list only the events whose delivery failed, and once it showed only the 50 newest, an older failure could not be found at all. It now has All, Failed (N) and Pending (N) links, carried in a `show` query parameter, so they work without the page's script library. Each filtered list keeps the 50-row limit and newest-first order, and lists an event once. It finds matching deliveries through `idx_deliveries_status` and looks their events up by ID, so its cost follows the matches, not the webhook's size. Replay returns to the list it was pressed in. The heading line says what a filter counts.

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