docs: document per-office Cloudflare env setup in README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -124,6 +124,25 @@ Copy the `.env.example` files and fill in the required values.
 npm run setup:env
 ```
 
+### Step 9a — Set the Cloudflare subdomain for this office
+
+After running `npm run setup:env`, open the two `.env` files and fill in this office's Cloudflare subdomain.
+
+**`apps/Frontend/.env`**
+```env
+VITE_CLOUDFLARE_HOST=yoursubdomain.mydentalofficemanagement.com
+```
+
+**`apps/Backend/.env`**
+```env
+CLOUDFLARE_HOST=yoursubdomain.mydentalofficemanagement.com
+FRONTEND_URLS=http://localhost:3000,http://yoursubdomain.mydentalofficemanagement.com,https://yoursubdomain.mydentalofficemanagement.com
+```
+
+Replace `yoursubdomain` with this office's actual subdomain (e.g. `summitdentalcare`).
+
+> If you skip this step, Cloudflare tunnel access will not work. LAN access still works without it.
+
 ### Step 10 — Set up the database
 
 ```sh