Converts a web page (by URL) or an uploaded HTML file to PDF.
GET /convert?url=https://example.com
POST /convert JSON: {"url": "https://example.com"}
POST /convert JSON: {"html": "<h1>Hi</h1>"}
POST /convert raw body with Content-Type: text/html
POST /convert multipart/form-data, HTML file in the "file" field
Options (query or JSON): format (A4), landscape (false),
margin (e.g. 1cm), scale (0.1–2), printBackground (true),
waitUntil (networkidle2), download (false — inline vs attachment).
Health check: GET /health