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/lib/constants.ts

@@ -0,0 +1,33 @@
+// ═══════════════════════════════════════════════════════════════
+// Server constants — all tunable magic numbers in one place
+// ═══════════════════════════════════════════════════════════════
+
+// --- Fight rules ---
+export const MAX_ROUNDS = 10
+export const KO_THRESHOLD = 0
+export const STARTING_HP = 200
+export const MAX_RESPONSE_BYTES = 10 * 1024 // 10KB
+export const MAX_ANSWER_LENGTH = 2000
+export const MAX_TRASH_TALK_LENGTH = 200
+
+// --- Elo ---
+export const ELO_K_FACTOR = 32           // K-factor for real fights
+export const ELO_K_FACTOR_MOCK = 12      // Dampened K-factor for mock/classic fights
+export const ELO_DIVISOR = 400           // Standard Elo divisor
+
+// --- Tier thresholds ---
+export const TIER_THRESHOLDS = [
+  { tier: 6, elo: 1900, wins: 40 },  // Legend
+  { tier: 5, elo: 1700, wins: 25 },  // Diamond
+  { tier: 4, elo: 1500, wins: 15 },  // Platinum
+  { tier: 3, elo: 1350, wins: 7 },   // Gold
+  { tier: 2, elo: 1200, wins: 3 },   // Silver
+  { tier: 1, elo: 0,    wins: 1 },   // Bronze
+] as const
+
+// --- Default challenge timeout ---
+export const DEFAULT_CHALLENGE_TIMEOUT_MS = 8000
+
+// --- Fight loop ---
+export const FIGHT_LOOP_INTERVAL_MS = 8000
+export const ELO_MATCHING_RANDOMNESS = 150