Published Column Fix

Replaced "Unpublished" label with created_at fallback display when published_at is nil. Investigation confirmed the EntryParser correctly extracts timestamps — the issue is feeds that genuinely lack date fields.

What Was Built

• Items index, item details, and source details views now show created_at in muted style when published_at is nil instead of "Unpublished"
• Added 4 controller tests covering published_at rendering and created_at fallback for both index and show actions
• Added 2 unit tests for EntryParser confirming timestamp extraction works for feeds with and without dates

Files Modified

• app/views/source_monitor/items/index.html.erb -- replaced "Unpublished" with created_at fallback in muted span
• app/views/source_monitor/items/_details.html.erb -- same fallback pattern in item detail and details hash
• app/views/source_monitor/sources/_details.html.erb -- same fallback pattern in source detail items table
• test/controllers/source_monitor/items_controller_test.rb -- added 4 new published_at/created_at tests
• test/lib/source_monitor/items/item_creator_test.rb -- added 2 EntryParser timestamp tests