fix: apply cbrt and cap to runestone formula to prevent AFK windfalls

apps/api/test/services/prestige.spec.ts

@@ -99,21 +99,27 @@ describe("isEligibleForPrestige", () => {
 
 describe("calculateRunestones", () => {
   it("calculates basic runestones formula", () => {
-    // floor(sqrt(4_000_000 / 1_000_000)) × 10 = floor(2) × 10 = 20
+    // floor(cbrt(4_000_000 / 1_000_000)) × 10 = floor(cbrt(4)) × 10 = 1 × 10 = 10
     const result = calculateRunestones({ totalGoldEarned: 4_000_000, prestigeCount: 0, purchasedUpgradeIds: [] });
-    expect(result).toBe(20);
+    expect(result).toBe(10);
   });
 
   it("applies echo runestone multiplier", () => {
-    // floor(sqrt(4) × 10) = 20; × 2 = 40
+    // floor(cbrt(4)) × 10 = 10; × 2 = 20
     const result = calculateRunestones({ totalGoldEarned: 4_000_000, prestigeCount: 0, purchasedUpgradeIds: [], echoRunestoneMultiplier: 2 });
-    expect(result).toBe(40);
+    expect(result).toBe(20);
   });
 
   it("applies purchased runestone upgrade multiplier", () => {
-    // With "runestones_1" purchased (multiplier 1.25): floor(20 × 1.25) = 25
+    // With "runestone_gain_1" purchased (multiplier 1.25): floor(10 × 1.25) = 12
     const result = calculateRunestones({ totalGoldEarned: 4_000_000, prestigeCount: 0, purchasedUpgradeIds: ["runestone_gain_1"] });
-    expect(result).toBeGreaterThan(20);
+    expect(result).toBe(12);
+  });
+
+  it("caps base runestones before multipliers", () => {
+    // cbrt(300_000_000 / 1_000_000) = cbrt(300) ≈ 6.67 → floor = 6 → 6 × 10 = 60, capped at 50
+    const result = calculateRunestones({ totalGoldEarned: 300_000_000, prestigeCount: 0, purchasedUpgradeIds: [] });
+    expect(result).toBe(50);
   });
 });