[IMP] website_sale_aplicoop: readable slug in the group order URL

Group order pages were published under their database id (`/eskaera/1`), which
says nothing to the member opening the link. `group.order` gains a `slug` field
and the public pages move to `/eskaera/<slug>` (e.g. `/eskaera/escola-fructuos`).

The slug is generated from the order name on creation, is unique, and can be
edited under the name on the form; emptying it regenerates it from the current
name. Values that would make the order unreachable are rejected: a plain number
(the legacy numeric URLs win that match) and the static routes served under
`/eskaera/` (`labels`, `save-order`, `i18n`, ...).

`/eskaera/<id>` and `/eskaera/<id>/checkout` are kept as redirects to their slug
URL, so the links already shared with members keep working. The AJAX endpoints
(`load-page`, `save-order`, `confirm`, ...) stay numeric: they never show up in
the address bar. Consequently the frontend now reads the order id from the
`data-order-id` attribute only, as the URL no longer carries it.

The post-migration script fills the slug of the orders that already existed.

Renaming the `/eskaera` prefix itself (`/escolas` for a schools deployment)
needs no code: a *308 Redirect / Rewrite* rule per public route in Website >
Configuration > Redirects serves the pages on the new prefix, rewrites the
links in the templates and redirects the old URLs, per website. Documented in
`readme/CONFIGURE.rst`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

website_sale_aplicoop/README.rst

@@ -95,6 +95,13 @@ Configuration
 2. Set pricing and availability per group order
 3. Assign products to categories used in group orders
 
+**Public URLs**
+
+1. Each group order is published under a readable slug: ``/eskaera/<slug>``
+2. The slug is generated from the order name and can be edited on the form (empty it to regenerate it)
+3. Links to ``/eskaera/<id>`` still work and redirect to the slug URL
+4. The ``/eskaera`` prefix can be renamed per website (e.g. ``/escolas``) with a **308 Redirect / Rewrite** rule in Website > Configuration > Redirects — see ``readme/CONFIGURE.rst``
+
 **Date & Time Validation**
 
 - ``start_date`` must be ≤ ``end_date`` (when both filled)