Phase 1: auth, room CRUD, WebSocket chat, PWA frontend

Invite-only FastAPI + SQLAlchemy(async) + Postgres backend (session-cookie
auth via CLI-provisioned accounts, open-room CRUD, single-instance /ws/chat)
and a React + Vite PWA frontend (login, room list, chat view). Backend tests
pass against a local Postgres DB. See README.md and backend/README.md for
setup, and ARCHITECTURE.md for the full phased design.

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

backend/app/models/membership.py

@@ -0,0 +1,31 @@
+import enum
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, Enum, ForeignKey, PrimaryKeyConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class RoomRole(str, enum.Enum):
+    owner = "owner"
+    admin = "admin"
+    member = "member"
+
+
+class RoomMembership(Base):
+    __tablename__ = "room_memberships"
+    __table_args__ = (PrimaryKeyConstraint("room_id", "user_id"),)
+
+    room_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("rooms.id"))
+    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"))
+    role: Mapped[RoomRole] = mapped_column(
+        Enum(RoomRole, name="room_role"), default=RoomRole.member, nullable=False
+    )
+    joined_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    room = relationship("Room", back_populates="memberships")
+    user = relationship("User")