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/models/user.py

@@ -0,0 +1,13 @@
+import uuid
+
+from fastapi_users.db import SQLAlchemyBaseUserTableUUID
+from sqlalchemy import String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class User(SQLAlchemyBaseUserTableUUID, Base):
+    __tablename__ = "users"
+
+    name: Mapped[str] = mapped_column(String(length=100), nullable=False)