feat(api): add OpenAPI documentation

README.md

@@ -64,7 +64,7 @@ npm install
 npm run dev
 ```
 
-The frontend is available at `http://127.0.0.1:3000`. The backend health endpoint is available at `http://127.0.0.1:8080/health`.
+The frontend is available at `http://127.0.0.1:3000`. The backend health endpoint is available at `http://127.0.0.1:8080/health`, and the interactive API documentation is available at `http://127.0.0.1:8080/docs`.
 
 ## Running with Docker Compose
 
@@ -86,6 +86,8 @@ The database volume is preserved by default. To remove it intentionally, run `do
 
 ## API Contract
 
+The OpenAPI 3.0 document is served at `http://127.0.0.1:8080/openapi.json`. Swagger UI is available at `http://127.0.0.1:8080/swagger/`, with `/docs` provided as a shortcut. The OpenAPI document is embedded in the backend binary; only the Swagger UI assets are loaded from a CDN.
+
 ### Convert a card number to an IBAN
 
 ```http
@@ -167,5 +169,5 @@ The test suite covers card validation and masking, the conversion service, the Z
 - The frontend normalizes Persian and Arabic digits and removes spaces or hyphens before submission.
 - SQLite is sufficient for this assessment and uses a single writer connection for predictable behavior.
 - Only the IBAN is exposed to the frontend. Account details, owner information, and raw ZarinHub responses remain internal.
-- Retries, Docker, OpenAPI, CI, dark mode, and production infrastructure are intentionally outside the project scope.
+- Retries, CI, dark mode, and production infrastructure are intentionally outside the project scope.
 - The interface uses the existing ZarinPal design system's font and logo assets. The font license is stored in `frontend/public/fonts`.