feat: add milestone prestige bonuses every 5th prestige

Every 5th prestige awards a scaling runestone windfall:
milestone_number * 25 stones (prestige 5 = 25, 10 = 50, 50 = 250, etc).
Shown in the ascension success message when non-zero.

apps/api/src/routes/prestige.ts

@@ -47,7 +47,7 @@ prestigeRouter.post("/", async (context) => {
     challengeCrystals = result.crystalsAwarded;
   }
 
-  const { newState, newPrestigeData, runestonesEarned } = buildPostPrestigeState(
+  const { newState, newPrestigeData, runestonesEarned, milestoneRunestones } = buildPostPrestigeState(
     state,
     characterName,
   );
@@ -81,6 +81,7 @@ prestigeRouter.post("/", async (context) => {
   return context.json({
     runestones: runestonesEarned,
     newPrestigeCount: newPrestigeData.count,
+    milestoneRunestones,
   });
 });