Add Phase 1 MVP: D&D text-adventure server

FastAPI + Postgres backend with fastapi-users auth, games/characters
CRUD, and a WebSocket chat endpoint where an x.ai Grok DM narrates
play, rolls dice, and maintains character sheets via tool calls.
React + Tailwind frontend covers registration/login, game list and
creation, participation-code join flow, character viewer, and a
real-time chat session view. Docker Compose skeleton (Postgres +
backend) included; Caddy/frontend container wiring is deferred to
the deploy milestone.

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

backend/app/llm/client.py

@@ -0,0 +1,18 @@
+from functools import lru_cache
+from pathlib import Path
+
+from openai import AsyncOpenAI
+
+from app.config import settings
+
+PROMPTS_DIR = Path(__file__).parent / "prompts"
+
+
+@lru_cache
+def get_dm_system_prompt() -> str:
+    return (PROMPTS_DIR / "dm_system_prompt.txt").read_text(encoding="utf-8")
+
+
+@lru_cache
+def get_llm_client() -> AsyncOpenAI:
+    return AsyncOpenAI(api_key=settings.xai_api_key, base_url=settings.xai_base_url)