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-extra.ts

@@ -1,11 +1,7 @@
 // Extra challenge prompts — Bitcoin/cypherpunk themed + general expansion
 // Adds ~75 prompts per type to reach 2,000+ total
 
-interface PromptEntry {
-  prompt: string
-  answers?: string[]
-  choices?: string[]
-}
+import type { PromptEntry } from './challenge-data.js'
 
 export const EXTRA_PROMPTS: Record<string, PromptEntry[]> = {
   speed_blitz: [