add dynamic routes plug and simplify router

Dynamic prefix rewriting for custom URL prefixes:
- DynamicRoutes plug rewrites /p/123 to /products/123
- 301 redirects from canonical to custom prefix
- Router simplified to 3 catch-all routes
- LiveSandboxHook for test process ownership

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/live_sandbox_hook.ex

@@ -0,0 +1,24 @@
+defmodule BerrypodWeb.LiveSandboxHook do
+  @moduledoc """
+  On-mount hook that allows LiveView processes to use the Ecto SQL sandbox.
+  Only active in test environment.
+  """
+
+  import Phoenix.LiveView
+  import Phoenix.Component
+
+  def on_mount(:default, _params, _session, socket) do
+    socket =
+      assign_new(socket, :phoenix_ecto_sandbox, fn ->
+        if connected?(socket), do: get_connect_info(socket, :user_agent)
+      end)
+
+    metadata = socket.assigns.phoenix_ecto_sandbox
+
+    if metadata do
+      Phoenix.Ecto.SQL.Sandbox.allow(metadata, Ecto.Adapters.SQL.Sandbox)
+    end
+
+    {:cont, socket}
+  end
+end