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_console.go

@@ -112,6 +112,7 @@ func (s *Server) registerConsole(router *gin.Engine) {
 	if s.payments != nil {
 		gm.GET("/catalog", s.consoleCatalog)
 		gm.POST("/catalog", s.consoleCreateProduct)
+		gm.POST("/catalog/reward", s.consoleSetReward)
 		gm.GET("/catalog/:id", s.consoleCatalogDetail)
 		gm.POST("/catalog/:id", s.consoleUpdateProduct)
 		gm.POST("/catalog/:id/archive", s.consoleArchiveProduct)