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/endpoint.ex

@@ -59,5 +59,6 @@ defmodule BerrypodWeb.Endpoint do
   plug Plug.MethodOverride
   plug Plug.Head
   plug Plug.Session, @session_options
-  plug BerrypodWeb.Router
+  plug BerrypodWeb.Plugs.Redirects
+  plug BerrypodWeb.Plugs.BrokenUrlTracker, router: BerrypodWeb.Router
 end