feat: set baseline scrape defaults for reliability and integration

[gildesmarais]

Summary

• commit 1: default block_images to true for POST /scrape baseline request behavior
• commit 2: align timeout defaults with html2rss/html2rss-web runtime budget (SCRAPE_TIMEOUT_SECONDS=25, wait_timeout_seconds=15)
• keep existing tiny_profile=true and headless=false baseline behavior
• update README and API contract unit coverage for the new defaults

Validation

• SMOKE_HOST_PORT=4110 make smoke (pass)