Initial commit: Backend API Phase 1a complete

- FastAPI backend with JWT auth
- PostgreSQL database schema
- Trees and Sessions CRUD APIs
- Export functionality (Markdown, Text, HTML)
- Docker setup for local development
- Alembic migrations

backend/app/models/__init__.py

@@ -0,0 +1,7 @@
+from .user import User
+from .team import Team
+from .tree import Tree
+from .session import Session
+from .attachment import Attachment
+
+__all__ = ["User", "Team", "Tree", "Session", "Attachment"]