More reliable caching and downloads

We made caching and downloads more reliable:

• If caching fails, you still get the screenshot. JSON responses include screenshot_url even then; cache_url appears only when caching succeeds.
• Async requests and response_type=empty skip the cache, and "cache": "false" in POST requests now disables caching.
• Resuming cached downloads is more reliable: Range and If-Range requests are handled correctly, and every cached file has a consistent ETag. HTML and Markdown downloads with Unicode characters report the correct file size.

Each new cached render now gets its own URL. Use the URL from the latest API response rather than expecting an old one to update.

See the caching guide and cache options.