refactor: extract magic numbers to named constants

Frontend: choreography selection ratios, morph trigger chances,
creator cameo/ultimate chances, dodge/counter probabilities.
Server: HP, K-factors, Elo divisor, tier thresholds, fight loop interval.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/scoring.ts

@@ -1,3 +1,4 @@
+import { ELO_DIVISOR, TIER_THRESHOLDS } from '../lib/constants.js'
 import type { Challenge } from './challenges.js'
 import { pick } from '../lib/utils.js'
 import { checkAnswer } from './answers.js'
@@ -523,7 +524,7 @@ export function calculateElo(
   loserElo: number,
   k: number = 32,
 ): { newWinnerElo: number; newLoserElo: number } {
-  const expectedWinner = 1 / (1 + Math.pow(10, (loserElo - winnerElo) / 400))
+  const expectedWinner = 1 / (1 + Math.pow(10, (loserElo - winnerElo) / ELO_DIVISOR))
   const expectedLoser = 1 - expectedWinner
 
   return {
@@ -534,13 +535,10 @@ export function calculateElo(
 
 // Tier calculation
 export function calculateTier(elo: number, wins: number): number {
-  if (elo >= 1900 && wins >= 40) return 6  // Legend
-  if (elo >= 1700 && wins >= 25) return 5  // Diamond
-  if (elo >= 1500 && wins >= 15) return 4  // Platinum
-  if (elo >= 1350 && wins >= 7) return 3   // Gold
-  if (elo >= 1200 && wins >= 3) return 2   // Silver
-  if (wins >= 1) return 1                   // Bronze
-  return 0                                  // Baby
+  for (const t of TIER_THRESHOLDS) {
+    if (elo >= t.elo && wins >= t.wins) return t.tier
+  }
+  return 0  // Baby
 }
 
 export const TIER_NAMES = ['BABY', 'BRONZE', 'SILVER', 'GOLD', 'PLATINUM', 'DIAMOND', 'LEGEND'] as const