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/plugs/dynamic_routes.ex

@@ -0,0 +1,101 @@
+defmodule BerrypodWeb.Plugs.DynamicRoutes do
+  @moduledoc """
+  Plug that enables dynamic route prefixes for products, collections, and orders.
+
+  When a shop owner customises their URL prefixes (e.g., /products → /p), this plug:
+
+  1. **Rewrites incoming paths** - If the URL uses a custom prefix (/p/123), rewrites
+     path_info to the canonical form (/products/123) so the router matches.
+
+  2. **Redirects old prefixes** - If the URL uses the old default prefix when a custom
+     one is set (/products/123 when prefix is "p"), issues a 301 redirect to /p/123.
+
+  This plug must run AFTER the Redirects plug (which handles trailing slashes and
+  explicit redirects) but BEFORE the router.
+
+  ## How it works
+
+  The R module maintains an ETS cache of custom prefixes loaded from Settings.
+  `R.prefix/1` returns the current prefix for a type, and `R.prefix_type_from_segment/1`
+  performs reverse lookups.
+
+  ## Example
+
+      # Settings: url_prefixes = %{"products" => "p", "collections" => "c"}
+
+      GET /p/123           → Rewrite to /products/123, router matches :product
+      GET /products/123    → Redirect 301 to /p/123
+      GET /c/art           → Rewrite to /collections/art, router matches :collection
+      GET /collections/art → Redirect 301 to /c/art
+
+  When no custom prefix is set, paths pass through unchanged.
+  """
+
+  import Plug.Conn
+  alias BerrypodWeb.R
+
+  @default_prefixes %{
+    products: "products",
+    collections: "collections",
+    orders: "orders"
+  }
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    case conn.path_info do
+      [first_segment | rest] when rest != [] ->
+        handle_prefixed_path(conn, first_segment, rest)
+
+      _ ->
+        conn
+    end
+  end
+
+  defp handle_prefixed_path(conn, segment, rest) do
+    prefix_type = R.prefix_type_from_segment(segment)
+    current_prefix = prefix_type && R.prefix(prefix_type)
+    default_prefix = prefix_type && @default_prefixes[prefix_type]
+
+    cond do
+      # No prefix type matched - not a dynamic route
+      is_nil(prefix_type) ->
+        conn
+
+      # Using the current (possibly custom) prefix - rewrite to default for router
+      segment == current_prefix and current_prefix != default_prefix ->
+        rewrite_path(conn, default_prefix, rest, segment)
+
+      # Using default prefix but custom is set - redirect to custom prefix
+      segment == default_prefix and current_prefix != default_prefix ->
+        redirect_to_custom(conn, current_prefix, rest)
+
+      # Using the default prefix and no custom set, or already canonical
+      true ->
+        conn
+    end
+  end
+
+  # Rewrite path_info so the router's static routes match
+  # Store original path in conn.private for LiveView to use
+  defp rewrite_path(conn, canonical_prefix, rest, original_prefix) do
+    conn
+    |> put_private(:original_prefix, original_prefix)
+    |> put_private(:original_request_path, conn.request_path)
+    |> Map.put(:path_info, [canonical_prefix | rest])
+  end
+
+  # 301 redirect to the canonical custom prefix
+  defp redirect_to_custom(conn, custom_prefix, rest) do
+    path = "/" <> Enum.join([custom_prefix | rest], "/")
+    location = append_query(path, conn.query_string)
+
+    conn
+    |> put_resp_header("location", location)
+    |> send_resp(301, "")
+    |> halt()
+  end
+
+  defp append_query(path, ""), do: path
+  defp append_query(path, qs), do: "#{path}?#{qs}"
+end