Fixed-Interval Jitter

What Was Built

• Wired fixed-interval scheduling path to use adjusted_interval_with_jitter instead of exact minutes, applying default ±10% jitter to prevent thundering herd
• Added 3 tests: default jitter applied, jitter_proc override respected, zero jitter_percent produces exact interval

Files Modified

• lib/source_monitor/fetching/feed_fetcher/adaptive_interval.rb -- fixed-interval else branch now converts to seconds and calls adjusted_interval_with_jitter
• test/lib/source_monitor/fetching/feed_fetcher_adaptive_interval_test.rb -- 3 new tests (9 total, all passing)