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/message.py

@@ -0,0 +1,31 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import BigInteger, CheckConstraint, ForeignKey, Text, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class Message(Base):
+    __tablename__ = "messages"
+    __table_args__ = (
+        CheckConstraint("sender_type IN ('player', 'dm', 'system')", name="ck_message_sender_type"),
+    )
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True, autoincrement=True)
+    game_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("games.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    sender_type: Mapped[str] = mapped_column(Text, nullable=False)
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id"), nullable=True
+    )
+    character_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("characters.id"), nullable=True
+    )
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        server_default=func.now(), nullable=False, index=True
+    )