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/api/router.py

@@ -0,0 +1,8 @@
+from fastapi import APIRouter
+from app.api.endpoints import auth, trees, sessions
+
+api_router = APIRouter()
+
+api_router.include_router(auth.router)
+api_router.include_router(trees.router)
+api_router.include_router(sessions.router)