feat(payments): edit the rewarded-ads config from the admin catalog page

The "watch for chips" rewarded payout and its per-day / per-hour caps live in the
shared payments.config row and had no admin surface — they could only be changed by
SQL, contrary to D32 (the rewarded rate is meant to be admin-configurable). Add a
"Rewarded ads" form on the /_gm/catalog page: RewardConfig / SetRewardConfig on the
service (non-negative validated), a setRewardConfig store writer (the singleton
config row), the consoleSetReward handler + POST /_gm/catalog/reward route, and the
form pre-filled from the current config. No migration — the columns already exist.

The reward rate is a config value, not a sellable catalog atom (so a "no-ads forever"
style product is still out of scope by D32/D33).

Test: an integration test sets the config, checks the page pre-fill, and refuses a
negative value.

backend/internal/server/handlers_admin_catalog.go

@@ -46,13 +46,32 @@ func (s *Server) consoleCatalog(c *gin.Context) {
 	// Order the list like the public offer: sales (chip packs) first, then the chip-exchange values,
 	// grouped and price-sorted the same way, so the console mirrors what a buyer sees.
 	payments.SortAdminCatalog(products)
-	view := adminconsole.CatalogView{ShowAll: showAll}
+	payout, daily, hourly, err := s.payments.RewardConfig(c.Request.Context())
+	if err != nil {
+		s.consoleError(c, err)
+		return
+	}
+	view := adminconsole.CatalogView{ShowAll: showAll, RewardPayout: payout, RewardDailyCap: daily, RewardHourlyCap: hourly}
 	for _, p := range products {
 		view.Products = append(view.Products, catalogRow(p))
 	}
 	s.renderConsole(c, "catalog", "catalog", "Catalog", view)
 }
 
+// consoleSetReward updates the rewarded-video config (chips per view plus the per-day and per-hour
+// caps) from the catalog page's rewarded-ads form. A blank field reads as 0; a negative value is
+// refused by the service.
+func (s *Server) consoleSetReward(c *gin.Context) {
+	payout, _ := strconv.Atoi(strings.TrimSpace(c.PostForm("reward_payout")))
+	daily, _ := strconv.Atoi(strings.TrimSpace(c.PostForm("reward_daily")))
+	hourly, _ := strconv.Atoi(strings.TrimSpace(c.PostForm("reward_hourly")))
+	if err := s.payments.SetRewardConfig(c.Request.Context(), payout, daily, hourly); err != nil {
+		s.renderConsoleMessage(c, "Update failed", err.Error(), catalogBack)
+		return
+	}
+	s.renderConsoleMessage(c, "Saved", "the rewarded-ads config was updated", catalogBack)
+}
+
 // catalogRow projects a product into its list row.
 func catalogRow(p payments.AdminProduct) adminconsole.ProductRow {
 	row := adminconsole.ProductRow{ID: p.ID.String(), Title: p.Title, Active: p.Active, Transacted: p.Transacted}