feat: scale to 1,330 challenge prompts

Add 526 new prompts across all 16 challenge types with heavy
Bitcoin/cypherpunk themes. Factual types include multiple-choice
options. Separate file for maintainability, merged at runtime.

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

server/src/engine/challenges.ts

@@ -1,3 +1,5 @@
+import { EXTRA_PROMPTS } from './challenges-extra.js'
+
 export interface Challenge {
   type: string
   label: string
@@ -979,6 +981,13 @@ const TEMPLATES: ChallengeTemplate[] = [
   },
 ]
 
+
+// Merge extra prompts into templates
+for (const t of TEMPLATES) {
+  const extras = EXTRA_PROMPTS[t.type]
+  if (extras) t.prompts.push(...extras)
+}
+
 // ═══════════════════════════════════════════════
 // Challenge selection + generation
 // ═══════════════════════════════════════════════