refactor(data): establish API contract guardrails

e2e/README.md

@@ -13,6 +13,7 @@ This project uses Playwright for end-to-end tests.
 ```bash
 pnpm test:e2e
 pnpm test:e2e:chrome
+pnpm test:e2e:mobile-smoke
 pnpm test:e2e:real
 pnpm test:e2e:prod
 ```
@@ -25,6 +26,10 @@ pnpm exec playwright install chromium
 
 For local development on macOS, `pnpm test:e2e:chrome` can reuse the installed Google Chrome.
 
+`pnpm test:e2e:mobile-smoke` runs the critical authentication, logout, token
+refresh and paid-image unlock paths with the Pixel 7 profile. CI runs this tier
+on every pull request and `dev` / `test` / `main` push.
+
 ## Environment overrides
 
 ```bash