add dead link monitoring for outgoing content links

Scans page blocks and nav items for broken URLs (internal via DB
lookup, external via HTTP HEAD). Daily Oban cron at 03:30, plus
on-demand checks when pages are saved. Admin UI tab on redirects
page with re-check, ignore, and clickable source links.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/config.exs

@@ -97,6 +97,7 @@ config :berrypod, Oban,
        {"0 3 * * *", Berrypod.Analytics.RetentionWorker},
        {"0 4 * * *", Berrypod.Orders.AbandonedCartPruneWorker},
        {"0 5 * * 1", Berrypod.Workers.RedirectPrunerWorker},
+       {"30 3 * * *", Berrypod.Workers.DeadLinkCheckerWorker},
        {"0 2 * * *", Berrypod.Newsletter.CleanupWorker},
        {"*/5 * * * *", Berrypod.Newsletter.ScheduledCampaignWorker}
      ]}