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>

test/berrypod_web/plugs/broken_url_tracker_test.exs

@@ -0,0 +1,26 @@
+defmodule BerrypodWeb.Plugs.BrokenUrlTrackerTest do
+  use BerrypodWeb.ConnCase, async: true
+
+  alias Berrypod.Redirects
+
+  setup do
+    Redirects.create_table()
+    :ok
+  end
+
+  test "records broken URL on 404", %{conn: conn} do
+    conn = get(conn, "/zz-nonexistent-path")
+
+    assert conn.status in [404, 500]
+
+    [broken_url] = Redirects.list_broken_urls()
+    assert broken_url.path == "/zz-nonexistent-path"
+    assert broken_url.recent_404_count == 1
+  end
+
+  test "skips static asset paths", %{conn: conn} do
+    get(conn, "/assets/missing-file.js")
+
+    assert Redirects.list_broken_urls() == []
+  end
+end