Import documentation: backend/README.md

Import the existing local project snapshot. This commit organizes existing files; it does not represent newly developed functionality.

backend/README.md

@@ -0,0 +1,30 @@
+# CShare Backend
+
+This is the local production-path API scaffold from `CONTINUE.md`. It uses Express 5 and TypeScript with local JSON metadata plus disk storage so upload, share, preview, download, dashboard, and auth flows work without external infrastructure.
+
+## Commands
+
+```sh
+npm install
+npm run dev
+npm run check
+npm test
+npm run build
+```
+
+The API listens on `http://localhost:4001` by default. Frontend requests use `VITE_API_BASE_URL`; in production, the frontend falls back to same-origin API calls when the variable is omitted.
+
+## Implemented Locally
+
+- `GET /health`
+- `POST /api/auth/register`, `POST /api/auth/login`, `POST /api/auth/refresh`, `POST /api/auth/logout`, `GET /api/auth/me`
+- `POST /api/v1/files/upload`
+- `GET /api/v1/files`, `GET /api/v1/files/:id`, `DELETE /api/v1/files/:id`
+- `GET /api/v1/links/:linkId`, `GET /api/v1/links/:linkId/download`
+- `GET /api/v1/links/:linkId/preview`
+- `GET /api/v1/dashboard/stats`
+- `GET /api/admin/overview`, `/api/admin/files`, `/api/admin/users`, `/api/admin/events` protected by `x-admin-key`
+
+## Production Adapters Still Needed
+
+The remaining roadmap items require provisioned services and credentials: PostgreSQL/Drizzle migrations, Redis/BullMQ, Cloudflare R2, Resend, Stripe, Sentry, PM2, Nginx, TLS, firewall rules, backups, and DNS. The local API keeps storage and persistence isolated so those adapters can replace the JSON/disk implementation without changing frontend routes.