Source Deletion Fix

Fixed 500 error when deleting sources by adding proper error handling for FK constraint violations from host app model extensions.

What Was Built

• Wrapped @source.destroy in begin/rescue with ActiveRecord::InvalidForeignKey catch
• Added handle_destroy_failure private method supporting both turbo_stream (error toast) and html (flash alert) responses
• FK violations now produce a user-friendly "Cannot delete — other records still reference this source" message instead of a 500 error
• Added 6 new tests covering successful deletion, FK violation handling, and both response formats

Files Modified

• app/controllers/source_monitor/sources_controller.rb -- wrapped destroy in error handling, added handle_destroy_failure method
• test/controllers/source_monitor/sources_controller_test.rb -- added 6 destroy action tests