Update README: API backend is now open source

The Rails API backend is now open source (MIT) at
https://github.com/nativeapptemplate/nativeapptemplateapi. Replace the
former "purchase the API source code" section with a link to the
public repo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dadachi

README.md

@@ -8,10 +8,9 @@ The iOS version is available here: [NativeAppTemplate-Free-iOS](https://github.c
 ## Overview
 
 NativeAppTemplate-Free-Android is configured to connect to `api.nativeapptemplate.com`.  
-You can purchase the source code for the backend server APIs, made with Ruby on Rails, that power `api.nativeapptemplate.com` ([API Docs](https://nativeapptemplate.com/api-docs/index.html)):
+The Rails 8.1 API backend that powers `api.nativeapptemplate.com` is open source (MIT):
 
-- [NativeAppTemplate-API (Solo)](https://nativeapptemplate.com/products/api-solo)  
-- [NativeAppTemplate-API (Team)](https://nativeapptemplate.com/products/api-team)
+- [nativeapptemplateapi](https://github.com/nativeapptemplate/nativeapptemplateapi) · [API Docs](https://nativeapptemplate.com/api-docs/index.html)
 
 ### Screenshots
 
@@ -88,9 +87,7 @@ Example: [50pcs NFC Cards Ntag215](https://www.amazon.com/dp/B087FRYY8S) (Amazon
 2. View the updated Number Tags status in the **Shop Detail** screen or on the **Number Tags Webpage**.  
 
 The **Number Tags Webpage** updates in real-time using Rails [Turbo](https://turbo.hotwired.dev).  
-This functionality is available in:  
-- [NativeAppTemplate-API (Solo)](https://nativeapptemplate.com/products/api-solo)  
-- [NativeAppTemplate-API (Team)](https://nativeapptemplate.com/products/api-team)  
+This functionality is part of the open-source [nativeapptemplateapi](https://github.com/nativeapptemplate/nativeapptemplateapi) backend.  
 
 > **Note:**  
 > The GIF above shows [MyTurnTag Creator for iOS](https://apps.apple.com/app/myturntag-creator/id1516198303) in action, which may behave slightly differently from **NativeAppTemplate-Free-Android**.