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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Align barcode-refresh diagram with actual session handling

The diagram currently shows a "Session Cache" read/write path, but /api/get/barcode does not use a cache layer: it calls getActiveGetSessionForUser, which re-authenticates via authenticatePin on each request (lib/get/server.ts) before fetching the barcode (app/api/get/barcode/route.ts). This mismatch can mislead maintainers/integrators into expecting cache-backed behavior and stale-session semantics that do not exist in production.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Fix fulfillment diagram to avoid implying scan-state returns QR data

In the "no drop" branch, the diagram says scan-state retrieves barcode payload and returns active state + payload, but GET /api/get/scan-state only returns status metadata (state, lastCheckedAt, expiresAt) and never returns barcode data; barcode payload is served by a separate endpoint (/api/get/barcode). This documentation error can cause client implementations to call the wrong endpoint and miss QR payloads.

Useful? React with 👍 / 👎.