Add nextjs migration logic

examples/supertokens-migration-nextjs/README.md

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+# Next.js SuperTokens Migration Example
+
+This example verifies Rownd-to-SuperTokens lazy migration through `@rownd/next`.
+
+It has two apps:
+
+- `backend`: Express + SuperTokens + `@supertokens-plugins/rownd-nodejs`
+- `frontend`: Next.js App Router + `@rownd/next`
+
+## Run
+
+Build the local SDK packages from the repo root first:
+
+```bash
+npm run build
+```
+
+Start SuperTokens Core locally, then run the backend:
+
+```bash
+cd backend
+cp .env.example .env
+npm install
+npm run dev
+```
+
+Set `ROWND_APP_KEY` and `ROWND_APP_SECRET` in `backend/.env` before starting the backend.
+
+Run the Next.js frontend:
+
+```bash
+cd frontend
+cp .env.example .env.local
+npm install
+npm run dev
+```
+
+Set `NEXT_PUBLIC_ROWND_APP_KEY` in `frontend/.env.local` to the same Rownd app key used by the backend.
+
+Open `http://localhost:3000`, sign up as a new Rownd user, and verify that the backend receives `POST /auth/plugin/rownd/migrate`.

examples/supertokens-migration-nextjs/backend/.env.example

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+SUPERTOKENS_CONNECTION_URI=http://localhost:3567
+# SUPERTOKENS_API_KEY=your_core_api_key   # only needed if you set one in supertokens-core
+
+ROWND_APP_KEY=your_rownd_app_key
+ROWND_APP_SECRET=your_rownd_app_secret
+
+PORT=3001
+WEBSITE_ORIGIN=http://localhost:3000
+
+# Origin of the Rownd Hub iframe. Migration fetches come from the Hub, not from
+# the host app. Use http://localhost:8787 when running Hub locally, or
+# https://hub.rownd.io for production.
+HUB_ORIGIN=http://localhost:8787

examples/supertokens-migration-nextjs/backend/.gitignore

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+node_modules
+.env