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/test/services/prestige.spec.ts

@@ -151,12 +151,12 @@ describe("calculateMilestoneBonus", () => {
     expect(calculateMilestoneBonus(5)).toBe(25);
   });
 
-  it("returns 50 at prestige 10", () => {
-    expect(calculateMilestoneBonus(10)).toBe(50);
+  it("returns 100 at prestige 10", () => {
+    expect(calculateMilestoneBonus(10)).toBe(100);
   });
 
-  it("returns 75 at prestige 15", () => {
-    expect(calculateMilestoneBonus(15)).toBe(75);
+  it("returns 225 at prestige 15", () => {
+    expect(calculateMilestoneBonus(15)).toBe(225);
   });
 });