backend_design.txt

The backend will be a Python Flask server with the following components:

1. Database:
   - Use SQLite for simplicity
   - Tables:
     - Projects (id, name, description, start_date, end_date)
     - Stages (id, project_id, name, description, start_date, end_date, status)
     - Tasks (id, stage_id, name, description, deadline, status)

2. API Endpoints:
   - GET /projects: Retrieve all projects
   - POST /projects: Create a new project
   - GET /projects/<project_id>: Retrieve a specific project
   - PUT /projects/<project_id>: Update a project
   - DELETE /projects/<project_id>: Delete a project
   
   - GET /projects/<project_id>/stages: Retrieve all stages for a project
   - POST /projects/<project_id>/stages: Create a new stage for a project
   - PUT /stages/<stage_id>: Update a stage
   - DELETE /stages/<stage_id>: Delete a stage
   
   - GET /stages/<stage_id>/tasks: Retrieve all tasks for a stage
   - POST /stages/<stage_id>/tasks: Create a new task for a stage
   - PUT /tasks/<task_id>: Update a task
   - DELETE /tasks/<task_id>: Delete a task

3. Business Logic:
   - Calculate progress percentages for projects and stages
   - Identify upcoming deadlines (within the next 7 days)
   - Sort and filter projects, stages, and tasks by various criteria

4. Data Validation:
   - Ensure date consistency (e.g., end dates after start dates)
   - Validate required fields and data types

5. Error Handling:
   - Proper HTTP status codes and error messages for various scenarios

6. Security:
   - Basic authentication for API access
   - Input sanitization to prevent SQL injection

The backend will serve the static HTML file for the frontend and handle all data operations through the API endpoints.