Phase 4a: deployment-provisioning choreography saga

Wire a full tenant deployment as one orchestrated, compensating saga:
mark → create droplet → wait active → register in inventory → link to
deployment → point DNS → activate. A failure anywhere rolls the whole
thing back — droplet destroyed, DNS reverted, deployment moved to
cancelled.

- New lifecycle state `provisioning`; deployments created via the
  provision path enter here and only reach `active` once the saga's
  ActivateDeployment step runs.
- Four new steps: MarkDeploymentProvisioning (owns the deployment's
  failure state), LinkDeploymentResource, PointDeploymentDns,
  ActivateDeployment.
- Provisioning.provision_deployment/2 assembles + starts the saga.
- DeploymentController: POST /deployments with provision:true creates
  in `provisioning` and kicks the saga (202); GET /deployments/:id now
  returns the provisioning saga + per-step progress.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/arcadia_cloud/provisioning.ex

@@ -148,6 +148,54 @@ defmodule ArcadiaCloud.Provisioning do
     })
   end
 
+  alias ArcadiaCloud.Provisioning.Steps
+
+  @doc """
+  Assembles + starts the full deployment-provisioning choreography saga
+  for a deployment that was created in the `provisioning` state.
+
+  Steps: mark → create droplet → wait active → register in inventory →
+  link to deployment → point DNS → activate. A failure anywhere rolls
+  the whole thing back (droplet destroyed, DNS reverted, deployment
+  moved to `cancelled`).
+
+  Required opts: :size, :image. Optional: :region (falls back to the
+  deployment's region), :dns_domain, :dns_record_name (falls back to the
+  deployment slug), :triggered_by.
+  """
+  def provision_deployment(deployment, opts \\ []) do
+    region = opts[:region] || deployment.region
+
+    inputs = %{
+      droplet_name: opts[:droplet_name] || "dep-#{deployment.slug}",
+      droplet_region: region,
+      droplet_size: opts[:size],
+      droplet_image: opts[:image],
+      droplet_tags: [
+        "deployment:#{deployment.id}",
+        "tenant:#{deployment.tenant_id}"
+      ],
+      dns_domain: opts[:dns_domain],
+      dns_record_name: opts[:dns_record_name] || deployment.slug
+    }
+
+    start_saga(%{
+      kind: "provision",
+      deployment_id: deployment.id,
+      triggered_by: opts[:triggered_by] || "manual",
+      step_modules: [
+        Steps.MarkDeploymentProvisioning,
+        Steps.CreateDroplet,
+        Steps.WaitDropletActive,
+        Steps.RegisterDroplet,
+        Steps.LinkDeploymentResource,
+        Steps.PointDeploymentDns,
+        Steps.ActivateDeployment
+      ],
+      inputs: inputs
+    })
+  end
+
   def get_saga(id), do: Repo.get(SagaRun, id)
   def get_saga!(id), do: Repo.get!(SagaRun, id)