Import documentation: PRODUCTION.md

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

PRODUCTION.md

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+# Production Notes
+
+## Build
+
+Frontend:
+
+```sh
+cd cshare-app
+npm ci
+npm run check
+```
+
+`npm run build` also creates `.gz` and `.br` siblings for compressible files in `dist/`.
+
+Backend:
+
+```sh
+cd backend
+npm ci
+npm run check
+NODE_ENV=production npm start
+```
+
+## Required Environment
+
+Set these before starting the backend in production:
+
+```sh
+NODE_ENV=production
+PORT=4001
+FRONTEND_URL=https://cshare.app
+PUBLIC_APP_URL=https://cshare.app
+JWT_ACCESS_SECRET=<64+ random chars>
+JWT_REFRESH_SECRET=<64+ random chars>
+ADMIN_API_KEY=<64+ random chars>
+PREVIEW_MAX_FILE_SIZE_BYTES=25000000
+DATA_DIR=/var/lib/cshare
+```
+
+Set `VITE_API_BASE_URL` for the frontend only when the API is not served from the same origin. If omitted in production, the frontend calls `window.location.origin`, which works with an Nginx reverse proxy that forwards `/api` to the backend.
+
+## Runtime
+
+- Serve `cshare-app/dist/` with Nginx or another static host.
+- Cache hashed files under `/assets/` aggressively.
+- Serve precompressed `.br` first and `.gz` second when the client supports them.
+- Do not cache `/index.html`; it must be revalidated so deploys pick up new asset hashes.
+- Proxy `/api`, `/health`, and file download routes to the backend.
+- Keep `backend/data/` outside the repo and backed up if using the local disk adapter.
+- Protect every `/api/admin/*` request with `x-admin-key: $ADMIN_API_KEY`.
+- Small safe file previews are served from `/api/v1/links/:linkId/preview`; files over `PREVIEW_MAX_FILE_SIZE_BYTES` remain download-only.
+- The admin API is disabled unless `ADMIN_API_KEY` is set.
+
+Example Nginx compression settings:
+
+```nginx
+brotli on;
+brotli_static on;
+gzip on;
+gzip_static on;
+gzip_types text/css application/javascript application/json image/svg+xml;
+```
+
+`brotli_static` requires an Nginx build with the Brotli module. If that module is unavailable, keep `gzip_static on` and serve the generated `.gz` files.
+
+The current backend is production-shaped but still uses local JSON and disk storage. For durable production use, replace it with the PostgreSQL, Redis, and R2 adapters described in `CONTINUE.md`.