Remove Default Scrape Limit

What Was Built

Removed the default per-source scrape limit (max_in_flight_per_source) from 25 to nil. Solid Queue's worker pool provides natural backpressure. Users who want a cap can still set config.scraping.max_in_flight_per_source explicitly.

Files Modified

• lib/source_monitor/configuration/scraping_settings.rb — DEFAULT_MAX_IN_FLIGHT changed from 25 to nil
• test/lib/source_monitor/scraping/enqueuer_test.rb — added nil-default rate-limit bypass test
• test/lib/source_monitor/scraping/bulk_result_presenter_test.rb — added nil/explicit limit message tests