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/templates/pages/catalog.gohtml

@@ -21,6 +21,15 @@
 <div><button type="submit">Add</button></div>
 </form>
 </section>
+<section class="panel"><h2>Rewarded ads (watch for chips)</h2>
+<p class="note">The chips a player earns per rewarded-video view (VK only), plus the per-day and per-hour caps that bound free chips — <strong>0 payout turns rewarded off</strong>. This is the shared reward config, not a product.</p>
+<form class="form col" method="post" action="/_gm/catalog/reward">
+<label>Chips per view <input type="number" name="reward_payout" min="0" value="{{.RewardPayout}}"></label>
+<label>Daily cap <input type="number" name="reward_daily" min="0" value="{{.RewardDailyCap}}"></label>
+<label>Hourly cap <input type="number" name="reward_hourly" min="0" value="{{.RewardHourlyCap}}"></label>
+<div><button type="submit">Save</button></div>
+</form>
+</section>
 <section class="panel"><h2>Products</h2>
 <p class="note">Showing {{if .ShowAll}}<strong>all</strong> products (active + archived) — <a href="/_gm/catalog">active only</a>{{else}}<strong>active</strong> products — <a href="/_gm/catalog?all=1">show all (incl. archived)</a>{{end}}.</p>
 <table class="list">