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/adminconsole/views.go

@@ -680,6 +680,12 @@ type CatalogView struct {
 	// ShowAll reports whether archived products are listed alongside active ones (the active/all
 	// toggle); it drives the toggle link and the list heading.
 	ShowAll bool
+	// RewardPayout / RewardDailyCap / RewardHourlyCap are the rewarded-video config (chips earned per
+	// view and the per-day / per-hour anti-abuse caps), shown in and edited from the page's
+	// rewarded-ads form. A 0 payout means rewarded is inert.
+	RewardPayout    int
+	RewardDailyCap  int
+	RewardHourlyCap int
 }
 
 // ProductRow is one product in the catalog list: its composition, prices, the archived flag