feat(housekeeping): Support for retrieving default Realm replication settings via API

Expose the default ScyllaDB replication settings (SimpleStrategy or
NetworkTopologyStrategy) configured in the Astarte instance. This allows
administrators to query the fallback parameters used during Realm creation.

Changes:
- Implement `RealmController.get_default_replication/2` to fetch and
  return replication config.
- Update Router to include the new `/realm-defaults/replication` path.
- Update OpenAPI specification with the new endpoint and
  `DefaultReplication` schema.
- Fix YAML indentation for existing response codes in `astarte_housekeeping_api.yaml`.

Signed-off-by: Eddy Babetto <eddy.babetto@secomind.com>

Author: eddbbt

apps/astarte_housekeeping/lib/astarte_housekeeping_web/controllers/realm_controller.ex

@@ -20,6 +20,7 @@ defmodule Astarte.HousekeepingWeb.RealmController do
   use Astarte.HousekeepingWeb, :controller
   use OpenApiSpex.ControllerSpecs
 
+  alias Astarte.Housekeeping.Config
   alias Astarte.Housekeeping.Realms
   alias Astarte.Housekeeping.Realms.Realm
   alias OpenApiSpex.{Reference, Schema}
@@ -171,6 +172,19 @@ defmodule Astarte.HousekeepingWeb.RealmController do
       unprocessable_entity: "Connected devices present"
     ]
 
+  operation :get_default_replication,
+    summary: "Get default Realm replication settings",
+    description: "Retrieves the default DB replication settings applied to new Realms.",
+    operation_id: "getDefaultReplication",
+    responses: [
+      ok:
+        {"Success", "application/json",
+         %Reference{"$ref": "#/components/schemas/DefaultReplication"}},
+      unauthorized: %Reference{"$ref": "#/components/responses/Unauthorized"},
+      forbidden: %Reference{"$ref": "#/components/responses/AuthorizationPathNotMatched"},
+      service_unavailable: "Astarte is unable to retrieve the replication settings."
+    ]
+
   def index(conn, _params) do
     with {:ok, realms} <- Realms.list_realms() do
       render(conn, "index.json", realms: realms)
@@ -224,6 +238,27 @@ defmodule Astarte.HousekeepingWeb.RealmController do
     end
   end
 
+  def get_default_replication(conn, _params) do
+    # TODO: this function needs to retrieve the default replication from the db
+    # to change once the appropriate pr has been merged
+    replication =
+      case Config.astarte_keyspace_replication_strategy!() do
+        :simple_strategy ->
+          %{
+            replication_class: :simple_strategy,
+            replication_factor: Config.astarte_keyspace_replication_factor!()
+          }
+
+        :network_topology_strategy ->
+          %{
+            replication_class: :network_topology_strategy,
+            datacenter_replication_factor: Config.astarte_keyspace_network_replication_map!()
+          }
+      end
+
+    render(conn, "replication.json", replication: replication)
+  end
+
   defp normalize_update_attrs(update_attrs) when is_map(update_attrs) do
     update_attrs
     |> Map.replace_lazy(:device_registration_limit, &normalize_integer_or_nil/1)

apps/astarte_housekeeping/lib/astarte_housekeeping_web/router.ex

@@ -28,6 +28,7 @@ defmodule Astarte.HousekeepingWeb.Router do
     pipe_through :api
 
     get "/version", VersionController, :show
+    get "/realm-defaults/replication", RealmController, :get_default_replication
 
     resources "/realms", RealmController,
       except: [:new, :edit, :update],

apps/astarte_housekeeping/lib/astarte_housekeeping_web/views/realm_view.ex

@@ -34,6 +34,10 @@ defmodule Astarte.HousekeepingWeb.RealmView do
     realm.realm_name
   end
 
+  def render("replication.json", %{replication: replication}) do
+    %{data: replication}
+  end
+
   def render("realm.json", %{realm: %Realm{replication_class: "SimpleStrategy"} = realm}) do
     %{
       data: %{

apps/astarte_housekeeping/priv/static/astarte_housekeeping_api.yaml

@@ -212,6 +212,55 @@ paths:
                   detail: Not found
       requestBody:
         $ref: "#/components/requestBodies/updateRealmBody"
+  /realm-defaults/replication:
+    get:
+      tags:
+        - realm
+      summary: Get default Realm replication settings
+      description: >-
+        Retrieves the default ScyllaDB replication settings configured for this Astarte instance.
+        These are the settings that Astarte will automatically enforce when a new Realm is
+        created without explicit replication parameters.
+      operationId: getDefaultReplication
+      security:
+        - JWT: []
+      responses:
+        "200":
+          description: Success
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  data:
+                    $ref: "#/components/schemas/DefaultReplication"
+        "401":
+          description: Authorization information is missing or invalid.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/GenericError"
+              example:
+                errors:
+                  detail: Unauthorized
+        "403":
+          description: Forbidden. The provided JWT token does not grant access to system configurations.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/GenericError"
+              example:
+                errors:
+                  detail: Forbidden
+        "503":
+          description: Service Unavailable. Astarte is unable to retrieve the replication settings.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/GenericError"
+              example:
+                errors:
+                  detail: "Service Unavailable: unable to fetch topology from ScyllaDB"
 components:
   securitySchemes:
     JWT:
@@ -345,6 +394,25 @@ components:
           f9jgaxW4oQV85enS/OJrrC9jU11agRc4bDv1h4s2t+ETWb4llTVk3HMIHbC3EvKJ
           VwIDAQAB
           -----END PUBLIC KEY-----
+    DefaultReplication:
+      type: object
+      required:
+        - replication_class
+      properties:
+        replication_class:
+          type: string
+          example: "NetworkTopologyStrategy"
+          description: "The default Replication Class used for new realms (e.g., SimpleStrategy or NetworkTopologyStrategy)."
+        replication_factor:
+          type: integer
+          example: 3
+          description: 'The default replication factor. Populated only if replication_class is "SimpleStrategy".'
+        datacenter_replication_factor:
+          type: object
+          example:
+            datacenter_1: 3
+            datacenter_2: 2
+          description: 'The default datacenter replication map. Populated only if replication_class is "NetworkTopologyStrategy".'
     GenericError:
       type: object
       required: