feat: add difficulty field to prompts and wire into fight pacing

Round 1-2 picks easy prompts, 3-4 medium, 5+ hard (when tagged).
Graceful fallback to any prompt when no difficulty-tagged prompts exist.

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

server/src/engine/challenge-data.ts

@@ -1,12 +1,14 @@
 // Challenge prompt data — separated from challenge logic
 
 export type PromptTheme = 'bitcoin' | 'conspiracy' | 'pc_culture' | 'bot_coding'
+export type PromptDifficulty = 'easy' | 'medium' | 'hard'
 
 export interface PromptEntry {
   prompt: string
   answers?: string[]
   choices?: string[]
   theme?: PromptTheme
+  difficulty?: PromptDifficulty
 }
 
 export interface ChallengeTemplate {