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/plugs/broken_url_tracker.ex

@@ -0,0 +1,34 @@
+defmodule BerrypodWeb.Plugs.BrokenUrlTracker do
+  @moduledoc """
+  Wraps the router to record 404s in the broken URLs table.
+
+  Works in dev mode too — Plug.Debugger intercepts exceptions before
+  error templates render, so we catch NoRouteError here, record it,
+  then re-raise so the normal error handling continues.
+  """
+
+  @behaviour Plug
+
+  def init(opts) do
+    router = Keyword.fetch!(opts, :router)
+    router_opts = router.init([])
+    {router, router_opts}
+  end
+
+  def call(conn, {router, router_opts}) do
+    router.call(conn, router_opts)
+  rescue
+    e in Phoenix.Router.NoRouteError ->
+      unless static_path?(conn.request_path) do
+        Berrypod.Redirects.record_broken_url(conn.request_path, 0)
+      end
+
+      reraise e, __STACKTRACE__
+  end
+
+  defp static_path?(path) do
+    String.starts_with?(path, "/assets/") or
+      String.starts_with?(path, "/images/") or
+      String.starts_with?(path, "/favicon")
+  end
+end