fix: [carbone-ee] enable Studio and template management

The charm had no ``hooks/init``, so Carbone started in its default
stateless mode: no Studio, no template management, and ``CARBONE_BIND``
defaulting to ``127.0.0.1`` (unreachable behind the web-proxy).

Add ``hooks/init`` to:
- bind on ``0.0.0.0`` so the ``web-proxy`` can reach the service ;
- serve the Studio and ``/carbone-studio.js`` (``CARBONE_STUDIO``) ;
- enable stateful template management (``CARBONE_TEMPLATE_MANAGEMENT``):
  stable 64-bit template IDs, versioning and the ``/template`` endpoints ;
- align the datastore/configstore ownership on the unprivileged
  ``carbone`` image user, otherwise the container crash-loops with
  ``EACCES`` on ``/app/config/config.json``.

Persist ``/app/config`` as a ``config-resources`` and set a
``docker-compose.stop_grace_period`` of 200s: Carbone flushes its
template metadata on graceful shutdown, and Docker's 10s default would
SIGKILL it and lose templates on redeploy.  Flush metadata every
5 minutes to shrink the hard-crash window.

Bump the image to ``full-5.15.1`` and add a local ``basic-deploy`` test.

Verified locally: ``/status`` 200, ``/carbone-studio.js`` 200,
``GET /templates`` 200, template upload with versioning plus render
from the stable template ID, and metadata survival across a container
recreation.

carbone/metadata.yml

@@ -1,7 +1,20 @@
-## From carbone/carbone-ee:full-5.4.2
-docker-image: docker.0k.io/carbone-ee:5.4.2
+## From carbone/carbone-ee:full-5.15.1
+docker-image: docker.0k.io/carbone-ee:5.15.1
+docker-compose:
+  ## Carbone flushes its template metadata to disk on graceful shutdown.
+  ## Docker's default 10s stop timeout is shorter than Carbone's shutdown
+  ## sequence (exitQuietPeriod + flush), so a plain redeploy would SIGKILL
+  ## it and lose the templates/versioning.  Keep this above Carbone's
+  ## exitDeadline (default 3 min).
+  stop_grace_period: 200s
 data-resources:
+  ## Templates and the template-management metadata store (metadata.db)
+  ## both live here, so they survive redeploys and are covered by backup.
   - /app/template
+config-resources:
+  ## Holds the optional Enterprise license (*.carbone-license) and the
+  ## authentication public key (key.pub).  See README.org.
+  - /app/config
 
 uses:
   web-proxy: