feat(offer): live catalog price list in the public offer, served by the render sidecar

Robokassa moderation requires the public offer to list every digital good with
its price. Move /offer/ off the static landing container to the render sidecar:
it splices the live catalog price list (§4.4) into the owner-edited
ui/legal/offer_ru.md and renders it with the shared ui/src/lib/offer.ts — one
renderer, no drift, always matching the current catalog with no redeploy.

- backend: /api/v1/internal/offer/pricing (internal, off the edge allow-list)
  projects the active catalog into two markdown tables — chip packs priced per
  rail (roubles / VK votes / Telegram Stars) and chip-priced values — through
  payments.Money so no float reaches the page. Cached in memory: warmed at boot,
  marked stale on every catalog mutation, so a served render issues no query.
- renderer: GET /offer/ fetches the tables and substitutes them at the
  <#pricing_template#> marker, then renders; offer_ru.md is baked into the image
  and marked is bundled from ui. GET /offer -> 301. Only /offer/ is edge-exposed.
- caddy: route /offer/ to the sidecar; drop the now-dead landing /offer/
  handlers and the vite emit-offer plugin.
- offer: fill §4.3 (the chip-payment wording) and drop the in-page back link.
- landing footer: a feedback link (the offer's Telegram contact) beside the
  offer link.
- docs (ARCHITECTURE, FUNCTIONAL +_ru, renderer README), CI /offer/ probe,
  unit + integration + node tests.

backend/internal/inttest/payments_catalog_test.go

@@ -4,6 +4,7 @@ package inttest
 
 import (
 	"context"
+	"strings"
 	"testing"
 
 	"github.com/google/uuid"
@@ -111,3 +112,44 @@ func TestPaymentsCatalogExcludesDeactivated(t *testing.T) {
 		t.Error("deactivated product must not appear in the storefront")
 	}
 }
+
+// TestOfferPricingReflectsCatalogEdits verifies the public-offer price list (§4.4) is projected from
+// the live catalog and its cache is invalidated on a catalog mutation: a newly created pack appears
+// with its per-rail prices, and archiving it through the service drops it from the next read.
+func TestOfferPricingReflectsCatalogEdits(t *testing.T) {
+	svc := newPaymentsService()
+	ctx := context.Background()
+	title := "OfferTest " + uuid.NewString()
+	id, err := svc.CreateProduct(ctx, payments.ProductInput{
+		Title: title,
+		Atoms: []payments.AtomLine{{Atom: "chips", Quantity: 50}},
+		Prices: []payments.PriceLine{
+			{Method: "direct", Currency: payments.CurrencyRUB, Amount: 20000},
+			{Method: "vk", Currency: payments.CurrencyVote, Amount: 30},
+			{Method: "telegram", Currency: payments.CurrencyStar, Amount: 100},
+		},
+	}, true)
+	if err != nil {
+		t.Fatalf("create pack: %v", err)
+	}
+
+	md, err := svc.OfferPricing(ctx)
+	if err != nil {
+		t.Fatalf("offer pricing: %v", err)
+	}
+	if row := "| " + title + " | 200.00 | 30 | 100 |"; !strings.Contains(md, row) {
+		t.Fatalf("offer pricing missing the new pack row %q\n%s", row, md)
+	}
+
+	// Archiving through the service marks the cache stale; the next read must reproject without it.
+	if err := svc.SetProductActive(ctx, id, false); err != nil {
+		t.Fatalf("archive: %v", err)
+	}
+	md, err = svc.OfferPricing(ctx)
+	if err != nil {
+		t.Fatalf("offer pricing after archive: %v", err)
+	}
+	if strings.Contains(md, title) {
+		t.Errorf("archived pack must drop from the offer pricing:\n%s", md)
+	}
+}