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/router.ex

@@ -275,21 +275,25 @@ defmodule BerrypodWeb.Router do
         {BerrypodWeb.PageEditorHook, :mount_page_editor},
         {BerrypodWeb.NewsletterHook, :mount_newsletter}
       ] do
-      live "/", Shop.Page, :home
-      live "/about", Shop.Page, :about
-      live "/delivery", Shop.Page, :delivery
-      live "/privacy", Shop.Page, :privacy
-      live "/terms", Shop.Page, :terms
-      live "/contact", Shop.Page, :contact
-      live "/collections/:slug", Shop.Page, :collection
-      live "/products/:id", Shop.Page, :product
-      live "/cart", Shop.Page, :cart
-      live "/search", Shop.Page, :search
-      live "/checkout/success", Shop.Page, :checkout_success
-      live "/orders", Shop.Page, :orders
-      live "/orders/:order_number", Shop.Page, :order_detail
+      # ┌─────────────────────────────────────────────────────────────────────┐
+      # │ Dynamic Shop Routes                                                  │
+      # │                                                                       │
+      # │ All shop pages are served by Shop.Page with runtime route resolution │
+      # │ via BerrypodWeb.R. This enables shop owners to customise URLs.       │
+      # │                                                                       │
+      # │ Single-segment pages (/:slug):                                       │
+      # │   /cart, /about, /contact, /search, /delivery, /privacy, /terms,     │
+      # │   /orders, /checkout/success, plus custom CMS pages                  │
+      # │                                                                       │
+      # │ Two-segment pages (/:prefix/:id_or_slug):                            │
+      # │   /products/:id, /collections/:slug, /orders/:number                 │
+      # │   (prefixes are customisable, e.g. /p/:id, /shop/:slug)              │
+      # │                                                                       │
+      # │ See: R.default_page_type/1, R.default_prefix_type/1, @page_modules   │
+      # └─────────────────────────────────────────────────────────────────────┘
 
-      # Catch-all for custom CMS pages — must be last
+      live "/", Shop.Page, :home
+      live "/:prefix/:id_or_slug", Shop.Page, :dynamic_prefix
       live "/:slug", Shop.Page, :custom_page
     end