feat: add theme system, improve creative scoring spam detection

Theme system (Section 5.1):
- Add PromptTheme type ('bitcoin' | 'conspiracy' | 'pc_culture' | 'bot_coding')
- Add optional theme field to PromptEntry interface
- pickChallenge now accepts optional theme bias, picks target theme with
  30/20/20/30 distribution, prefers themed prompts when available
- challenges-extra.ts now imports PromptEntry type from challenge-data.ts

Creative scoring improvements (Section 7.2):
- Detect repeated phrases via trigram analysis (>25% duplicate = spam)
- Detect question echo (answer copies prompt back)
- Detect all-caps spam (>80% uppercase letters)
- Detect punctuation-only spam (<30% letter content)
- Minimum word count threshold (< 3 words = low score)
- 5 new tests: repeated phrases, question echo, all-caps, short answer, legit short

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

server/src/engine/challenges.ts

@@ -1,4 +1,4 @@
-import { TEMPLATES, type ChallengeTemplate, type PromptEntry } from './challenge-data.js'
+import { TEMPLATES, type ChallengeTemplate, type PromptEntry, type PromptTheme } from './challenge-data.js'
 import { EXTRA_PROMPTS } from './challenges-extra.js'
 import { pick } from '../lib/utils.js'
 
@@ -14,7 +14,7 @@ export interface Challenge {
   displayPrompt?: string
 }
 
-export type { ChallengeTemplate, PromptEntry }
+export type { ChallengeTemplate, PromptEntry, PromptTheme }
 
 
 // Merge extra prompts into templates
@@ -36,7 +36,25 @@ function shuffleArray<T>(arr: T[]): T[] {
   return s
 }
 
-export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | null): Challenge {
+// Target theme distribution: 30% bitcoin, 20% conspiracy, 20% pc_culture, 30% bot_coding
+const THEME_WEIGHTS: Record<PromptTheme, number> = {
+  bitcoin: 0.3,
+  conspiracy: 0.2,
+  pc_culture: 0.2,
+  bot_coding: 0.3,
+}
+
+function pickTheme(): PromptTheme | undefined {
+  const roll = Math.random()
+  let cumulative = 0
+  for (const [theme, weight] of Object.entries(THEME_WEIGHTS)) {
+    cumulative += weight
+    if (roll < cumulative) return theme as PromptTheme
+  }
+  return undefined
+}
+
+export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | null, themeBias?: PromptTheme): Challenge {
   let available = TEMPLATES.filter(t => !usedTypes.has(t.type))
   if (available.length === 0) available = TEMPLATES
 
@@ -52,7 +70,8 @@ export function pickChallenge(usedTypes: Set<string>, _arenaModifier: string | n
   }
 
   const template = pick(pool)
-  return templateToChallenge(template)
+  const targetTheme = themeBias || pickTheme()
+  return templateToChallenge(template, targetTheme)
 }
 
 /** Ranked challenge: no multiple choice, only harder creative/open-ended prompts */
@@ -83,8 +102,14 @@ export function pickRankedChallenge(usedTypes: Set<string>): Challenge {
   }
 }
 
-function templateToChallenge(template: ChallengeTemplate): Challenge {
-  const entry = pick(template.prompts)
+function templateToChallenge(template: ChallengeTemplate, targetTheme?: PromptTheme): Challenge {
+  // Prefer prompts matching target theme if any are tagged
+  let prompts = template.prompts
+  if (targetTheme) {
+    const themed = template.prompts.filter(p => p.theme === targetTheme)
+    if (themed.length > 0) prompts = themed
+  }
+  const entry = pick(prompts)
 
   // Determine choices
   let choices: string[] | undefined