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/docker-compose.yml

@@ -0,0 +1,22 @@
+version: '3.8'
+
+services:
+  db:
+    image: postgres:16-alpine
+    container_name: decision_tree_db
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+      POSTGRES_DB: decision_tree
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+
+volumes:
+  postgres_data: