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/payments/service_intake.go

@@ -157,6 +157,22 @@ func (s *Service) RewardPayout(ctx context.Context, cxt Context, present []Sourc
 	return payout, err
 }
 
+// RewardConfig reads the rewarded-video config for the admin editor: the payout (chips per view) and
+// the per-day and per-hour caps.
+func (s *Service) RewardConfig(ctx context.Context) (payout, dailyCap, hourlyCap int, err error) {
+	return s.store.rewardConfig(ctx)
+}
+
+// SetRewardConfig updates the rewarded-video config (the payout and the two caps). All three must be
+// non-negative; a 0 payout leaves rewarded inert. It is not a catalog product, so it does not mark the
+// offer stale.
+func (s *Service) SetRewardConfig(ctx context.Context, payout, dailyCap, hourlyCap int) error {
+	if payout < 0 || dailyCap < 0 || hourlyCap < 0 {
+		return errors.New("payments: reward payout and caps must be non-negative")
+	}
+	return s.store.setRewardConfig(ctx, payout, dailyCap, hourlyCap)
+}
+
 // CreditReward credits a rewarded-video view's chips to the VK segment, client-attested (VK Mini App
 // ads expose no server verify — the client's watch result is trusted for an honest user; a forger who
 // skips the ad and calls the endpoint is bounded by the config daily cap). It is idempotent on the