add URL redirects with ETS-cached plug, broken URL tracking, and admin UI

Redirects context with redirect/broken_url schemas, chain flattening,
ETS cache for fast lookups in the request pipeline. BrokenUrlTracker
plug logs 404s. Auto-redirect on product slug change via upsert_product
hook. Admin redirects page with active/broken tabs, manual create form.
RedirectPrunerWorker cleans up old broken URLs. 1227 tests passing.

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

priv/repo/migrations/20260225073522_create_redirects_and_broken_urls.exs

@@ -0,0 +1,37 @@
+defmodule Berrypod.Repo.Migrations.CreateRedirectsAndBrokenUrls do
+  use Ecto.Migration
+
+  def change do
+    create table(:redirects, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :from_path, :string, null: false
+      add :to_path, :string, null: false
+      add :status_code, :integer, default: 301
+      add :source, :string, null: false
+      add :confidence, :float
+      add :hit_count, :integer, default: 0
+
+      timestamps()
+    end
+
+    create unique_index(:redirects, [:from_path])
+    create index(:redirects, [:source])
+
+    create table(:broken_urls, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :path, :string, null: false
+      add :prior_analytics_hits, :integer, default: 0
+      add :recent_404_count, :integer, default: 1
+      add :first_seen_at, :utc_datetime, null: false
+      add :last_seen_at, :utc_datetime, null: false
+      add :status, :string, default: "pending"
+      add :resolved_redirect_id, references(:redirects, type: :binary_id, on_delete: :nilify_all)
+
+      timestamps()
+    end
+
+    create unique_index(:broken_urls, [:path])
+    create index(:broken_urls, [:status])
+    create index(:broken_urls, [:prior_analytics_hits])
+  end
+end