Sitemap uploads — add continue_on_failure option

[a-klos]

Goal
Allow sitemap crawls to skip failed pages instead of aborting the whole crawl.

Changes

• Backend

  • Parse + normalize continue_on_failure in _parse_sitemap_loader_parameters (accept bool and "true"/"false" strings).
  • Default continue_on_failure=True in aextract_content when omitted.

• Frontend

  • Add “Continue on failure” checkbox to sitemap upload UI (default checked).
  • Send continueOnFailure in API payload; update SitemapConfig.

• Docs & i18n

  • Document continue_on_failure (boolean, default true) in README sitemap section.
  • Add EN/DE translations (label + tooltip).

• Tests

  • Cover normalization, defaulting, and payload passing.

Acceptance criteria

• Default behavior continues crawl on failures.
• Setting false aborts on first failing page.
• All tests pass.