fix: add generation guard to refresh, preserve image on transient failure, document env vars

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -137,6 +137,8 @@ The database will be automatically initialized and populated with games from `ga
    DB_PATH=./data/jackbox.db
    JWT_SECRET=your-secret-jwt-key
    ADMIN_KEY=admin123
+   VOTE_APP_URL=https://vote-app.example.com
+   VOTE_APP_API_KEY=your-vote-app-api-key
    ```
 
 4. **Start the backend server**
@@ -184,6 +186,13 @@ If `admins.json` is not found, the app falls back to the `ADMIN_KEY` environment
 
 To use a custom path for the admins file, set `ADMIN_CONFIG_PATH` in your environment.
 
+### Sticker Wall Preview
+
+The backend proxies sticker wall images from the vote-app. Set these in `backend/.env`:
+
+- `VOTE_APP_URL` — base URL of the vote-app (e.g. `https://vote-app.example.com`)
+- `VOTE_APP_API_KEY` — API key for the sticker wall render endpoint
+
 For Docker, uncomment the volume mount in `docker-compose.yml`:
 ```yaml
 - ./backend/config/admins.json:/app/config/admins.json:ro