feat(arcade): migrate backend from NestJS to Hono/Bun

Replace @arcade/backend (NestJS) with @arcade/server (Hono/Bun).
Same two endpoints, no auth required (public game generator):
- POST /api/games/generate — AI game generation (Gemini, Claude, GPT)
- POST /api/games/submit  — Community game submission via GitHub PR
- GET  /health            — Health check

This removes the last remaining NestJS backend from the monorepo.
NestJS is now completely gone — all servers use Hono + Bun.

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

scripts/generate-env.mjs

@@ -282,9 +282,9 @@ const APP_CONFIGS = [
 		},
 	},
 
-	// Arcade Backend (NestJS)
+	// Arcade Server (Hono/Bun)
 	{
-		path: 'games/arcade/apps/backend/.env',
+		path: 'games/arcade/apps/server/.env',
 		vars: {
 			NODE_ENV: () => 'development',
 			PORT: (env) => env.MANA_GAMES_BACKEND_PORT || '3011',