Update Architecture Diagram

As a developer, I want to update the Application Architecture Diagram in the README.md file to include a professional's feedback, so that the diagram is clearer and more accurate.

Acceptance criteria:

• Arrows should be bidirectional to represent all possible interactions between components in the application
• API should be represented by a cloud
• Database should be represented by stacked cylinders
• User should be represented by a user icon
• Other components should be represented with a rectangular box