Add architecture diagram and GET workflow diagrams to README

[darthnithin]

Motivation

• Provide a high-level architecture diagram to clarify how the Next.js app, NextAuth, API routes, the GET adapter layer, and Prisma/Postgres interact.
• Document the GET integration workflows to make account linking, barcode refresh, and fulfillment completion behavior explicit for maintainers and integrators.

Description

• Add a Mermaid flowchart showing the system architecture and relationships between UI, Auth, APIs, GET adapter, Prisma client, and the database.
• Add three Mermaid sequence diagrams illustrating the GET workflows: Account Linking, Barcode Refresh, and Fulfillment Completion.
• Update the README.md to include the new diagrams and section headers without changing application code or runtime behavior.

Testing

• Run npm run lint against the repository and the linter completed successfully.
• No automated unit tests were changed or required for this documentation-only update and CI behavior is expected to be unaffected.

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Codex Task