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/components/layouts/admin.html.heex

@@ -118,6 +118,14 @@
               <.icon name="hero-envelope" class="size-5" /> Email
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/redirects"}
+              class={admin_nav_active?(@current_path, "/admin/redirects")}
+            >
+              <.icon name="hero-arrow-uturn-right" class="size-5" /> Redirects
+            </.link>
+          </li>
         </ul>
       </nav>