Error Handling Safety Net

What Was Built

• Split rescue in update_source_state! so DB update errors propagate as exceptions while broadcast failures are logged and swallowed
• Added ensure block to FetchRunner#run that resets fetch_status from "fetching" to "failed" on any exit path
• Added per-item begin/rescue in FollowUpHandler#call so a single enqueue failure doesn't block other items or mark_complete!
• 5 new tests covering: DB error propagation, broadcast error swallowing, ensure status reset, partial enqueue failure resilience, full enqueue failure resilience

Files Modified

• lib/source_monitor/fetching/fetch_runner.rb -- split rescue in update_source_state!, added ensure block to #run
• lib/source_monitor/fetching/completion/follow_up_handler.rb -- added per-item rescue in #call
• test/lib/source_monitor/fetching/fetch_runner_test.rb -- 3 new tests for error handling
• test/lib/source_monitor/fetching/completion/follow_up_handler_test.rb -- new file, 2 tests