balance: smooth prestige income cliff, quadratic milestones, exponential combat scaling (#170, #171)

Reduce income_10 cost 30k→22.5k and income_11 80k→60k (25% cut each)
to ease the late-prestige runestone cliff without collapsing the timeline.

Change prestige milestone bonus from linear (n×25) to quadratic (n²×25)
so high-prestige milestones feel meaningful (P100 = 10k stones).

Replace linear prestige combat multiplier (1 + count×0.1) with exponential
(4^count) in both the tick engine and server-side boss route. Without this
the final boss (2×10^145 HP) was unreachable by ~112 orders of magnitude;
base-4 makes it achievable around P190, consistent with the 6-month target.

apps/api/src/data/prestigeUpgrades.ts

@@ -96,7 +96,7 @@ export const defaultPrestigeUpgrades: Array<PrestigeUpgrade> = [
     id:             "income_10",
     multiplier:     200,
     name:           "Eternal Rune I",
-    runestonesCost: 30_000,
+    runestonesCost: 22_500,
   },
   {
     category: "income",
@@ -105,7 +105,7 @@ export const defaultPrestigeUpgrades: Array<PrestigeUpgrade> = [
     id:             "income_11",
     multiplier:     500,
     name:           "Eternal Rune II",
-    runestonesCost: 80_000,
+    runestonesCost: 60_000,
   },
   // ── Click Power ───────────────────────────────────────────────────────────
   {