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/server/handlers.go

@@ -74,6 +74,9 @@ func (s *Server) registerRoutes() {
 		u.POST("/wallet/buy", s.handleWalletBuy)
 		// A rewarded-video credit (VK ads): client-attested + a config daily cap.
 		u.POST("/wallet/reward", s.handleWalletReward)
+		// The public-offer price list (§4.4) as markdown, for the render sidecar that serves the
+		// /offer/ page. Internal (off the edge allow-list); called by the renderer, not the gateway.
+		s.internal.GET("/offer/pricing", s.handleOfferPricing)
 	}
 	if s.payments != nil {
 		// The money order endpoint dispatches by rail (direct → Robokassa, vk → VK); an