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/store_intake.go

@@ -413,6 +413,18 @@ func (s *Store) rewardConfig(ctx context.Context) (payout, dailyCap, hourlyCap i
 	return int(cfg.RewardedPayoutChips), int(cfg.RewardDailyCap), int(cfg.RewardHourlyCap), nil
 }
 
+// setRewardConfig updates the rewarded payout and the per-day and per-hour caps on the singleton
+// config row (no WHERE — the table holds exactly one row). Non-negativity is validated by the caller
+// and also enforced by the config CHECK constraints.
+func (s *Store) setRewardConfig(ctx context.Context, payout, dailyCap, hourlyCap int) error {
+	if _, err := s.db.ExecContext(ctx,
+		`UPDATE payments.config SET rewarded_payout_chips=$1, reward_daily_cap=$2, reward_hourly_cap=$3`,
+		payout, dailyCap, hourlyCap); err != nil {
+		return fmt.Errorf("payments: set reward config: %w", err)
+	}
+	return nil
+}
+
 // creditReward credits a rewarded-video view's chips to the funded segment, client-attested (VK Mini
 // App ads expose no server verify). It reads the payout and daily cap from config: a 0 payout
 // (unconfigured) or a reached cap credits nothing. It is idempotent on the client nonce (dedup on the