Auto-generate an adventure outline when no adventure text is provided

When a player creates a game without pasting their own adventure, the
DM previously improvised turn-by-turn with no overall arc, ending, or
calibrated difficulty. Now the setup-chat interview generates a full
outline (hook, three-to-four-part structure, milestone leveling,
explicit ending condition, GM guidance) as a follow-up LLM call once
genre/length/experience are known, and feeds it through the existing
adventure_text -> RAG ingestion -> "follow strictly" pipeline exactly
like a pasted adventure. Falls back silently to today's improvised
behavior if generation fails.

Verified end-to-end in the browser: setup interview -> outline
generated and previewed -> game created -> RAG chunks ingested ->
has_adventure true -> cascade-cleaned on delete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/llm/client.py

@@ -18,6 +18,11 @@ def get_game_setup_prompt() -> str:
     return (PROMPTS_DIR / "game_setup_prompt.txt").read_text(encoding="utf-8")
 
 
+@lru_cache
+def get_adventure_outline_prompt() -> str:
+    return (PROMPTS_DIR / "adventure_outline_prompt.txt").read_text(encoding="utf-8")
+
+
 @lru_cache
 def get_llm_client() -> AsyncOpenAI:
     if settings.llm_provider == "lmstudio":