tracking: TrackEvent platform type + custom-track island; bump submodules

- platform/event: TrackEvent (track.event) free-form storefront custom
  events, wired into KnownTypes + the drift-guard surface
- data/templates/header.gohtml: emit the custom-track island
- backoffice @ b4e6244 (cart upsell catalog caching, custom-track,
  round-2 editor polish)
- go-graph-cms-v2 @ e4cc6d1 (strip-extras failure reporting,
  custom-track island entry)
- slask-tracking @ ed18ed6 (custom-stats analysis + TrackEvent)

event/event.go

@@ -92,6 +92,15 @@ const (
 	// `track.*` RabbitMQ routing pattern by construction.
 	TrackPromotionView  Type = "track.promotion_view"
 	TrackPromotionClick Type = "track.promotion_click"
+	// TrackEvent — free-form named storefront event (custom analytics
+	// signals the storefront wires ad hoc: open_cart, plp_sort_change,
+	// plp_filter_change, …). Producer: lib/track.ts trackEvent() beacons /
+	// the data-track attribute island (POST /api/track/event). Consumer:
+	// slask-tracking (pkg/view CustomEvent → pkg/customstats) — per-name
+	// counters for the tracking read surface. Payload is the wire
+	// CustomEvent (name + flat string props). Browser-beacon path only,
+	// like the promotion pair.
+	TrackEvent Type = "track.event"
 
 	// Newsletter campaign attribution (review 3.8). Producer: the order
 	// service's MailerSend webhook receiver (cmd/order/newsletter-webhook.go)
@@ -207,7 +216,7 @@ var KnownTypes = []Type{
 	TrackSession, TrackSearch, TrackClick, TrackCart, TrackPurchase,
 	TrackImpression, TrackAction, TrackPageView, TrackPageEngaged,
 	TrackPageAction, TrackExperiment,
-	TrackPromotionView, TrackPromotionClick,
+	TrackPromotionView, TrackPromotionClick, TrackEvent,
 	// Track* constants without a browser beacon: consumed over AMQP only,
 	// but the drift-guard test requires every declared Track* constant to
 	// appear here so the HTTP ?event_types= surface stays exhaustive.