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>

priv/repo/migrations/20260301100229_create_dead_links.exs

@@ -0,0 +1,20 @@
+defmodule Berrypod.Repo.Migrations.CreateDeadLinks do
+  use Ecto.Migration
+
+  def change do
+    create table(:dead_links, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :url, :string, null: false
+      add :url_type, :string, null: false, default: "external"
+      add :status, :string, null: false, default: "broken"
+      add :http_status, :integer
+      add :error, :string
+      add :last_checked_at, :utc_datetime, null: false
+
+      timestamps()
+    end
+
+    create unique_index(:dead_links, [:url])
+    create index(:dead_links, [:status])
+  end
+end