balance: increase runestone yield 50% per prestige (#166)

Raise runestonesPerPrestigeLevel from 10 to 15. Early-game players
were earning only 10-20 runestones per prestige, making the upgrade
shop feel out of reach. This boost helps mid-game without affecting
the cap behaviour (cbrt formula still prevents AFK windfalls).

apps/api/src/services/prestige.ts

@@ -15,7 +15,7 @@ import type {
 } from "@elysium/types";
 
 const basePrestigeGoldThreshold = 1_000_000;
-const runestonesPerPrestigeLevel = 10;
+const runestonesPerPrestigeLevel = 15;
 const milestoneInterval = 5;
 const milestoneRunestonesPerInterval = 25;