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>

lib/berrypod_web/router.ex

@@ -229,6 +229,7 @@ defmodule BerrypodWeb.Router do
       live "/providers/:id/edit", Admin.Providers.Form, :edit
       live "/settings", Admin.Settings, :index
       live "/settings/email", Admin.EmailSettings, :index
+      live "/redirects", Admin.Redirects, :index
     end
 
     # Theme editor: admin root layout but full-screen (no sidebar)