feat: ranked queue Elo-bracket matchmaking and harder prompts

Elo-bracket matchmaking: prefer ±200, widen by 100 every 15s of waiting.
Add pickRankedChallenge() that filters to creative/open-ended only, never
multiple choice. Show ranked queue status, wait estimate, and "HARDER
PROMPTS" notice on JoinBoutPage.

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

server/src/engine/challenges.ts

@@ -1011,6 +1011,34 @@ export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | n
   return templateToChallenge(template)
 }
 
+/** Ranked challenge: no multiple choice, only harder creative/open-ended prompts */
+export function pickRankedChallenge(usedTypes: Set<string>): Challenge {
+  let available = TEMPLATES.filter(t => !usedTypes.has(t.type))
+  if (available.length === 0) available = TEMPLATES
+
+  // Filter to creative-only (open-ended, harder) — no factual multiple choice
+  const creative = available.filter(t => t.scoring === 'creative')
+  const pool = creative.length > 0 ? creative : available
+
+  const template = pool[Math.floor(Math.random() * pool.length)]
+
+  // Pick a prompt that has no choices array (open-ended)
+  const openPrompts = template.prompts.filter(p => !p.choices)
+  const prompts = openPrompts.length > 0 ? openPrompts : template.prompts
+  const entry = prompts[Math.floor(Math.random() * prompts.length)]
+
+  return {
+    type: template.type,
+    label: template.label,
+    prompt: entry.prompt,
+    answers: entry.answers,
+    choices: undefined, // Never multiple choice in ranked
+    timeout_ms: template.timeout_ms,
+    scoring: template.scoring,
+    baseDamage: template.baseDamage,
+  }
+}
+
 function templateToChallenge(template: ChallengeTemplate): Challenge {
   const entry = template.prompts[Math.floor(Math.random() * template.prompts.length)]