Phase 2: first real DO write step — CreateDropletSnapshot

DigitalOcean.Client write methods:
- create_droplet_snapshot/3 — POST a snapshot action (async)
- get_droplet_action/3      — poll action status
- list_droplet_snapshots/2  — snapshots for a droplet
- delete_snapshot/2         — DELETE (used by compensation)
All use the "provisioning" token purpose.

Steps.CreateDropletSnapshot — the first saga step that touches real
infra:
- execute: deterministic snapshot name (arcadia-snap-<droplet>-<saga8>);
  checks context for a prior snapshot_id, then checks DO for a snapshot
  already carrying that name (crash-between-post-and-save recovery),
  then posts the action, polls to completion, finds the resulting
  snapshot, records snapshot_id + snapshot_name in context.
- compensate: deletes the snapshot; treats HTTP 404 as success.

Provisioning.snapshot_droplet/2 — convenience saga starter.

Two DO eventual-consistency gotchas surfaced + handled:
- After a snapshot action reports "completed", the snapshot lags a few
  seconds before appearing in /droplets/:id/snapshots. The step now
  retries the lookup (find_snapshot_with_retry, 12x5s) instead of
  failing with :snapshot_not_found_after_completion.
- Deletion has the same lag the other way — a deleted snapshot lingers
  in the listing briefly. compensate just trusts the DELETE 2xx/404;
  no post-delete verification needed.

Live smoke verified end-to-end against holyspiritbraypark.com:
[CreateDropletSnapshot, Fail] saga — the step created real snapshot
229305609, the Fail step triggered compensation, compensation deleted
the snapshot. Final: saga rolled_back, ledger
[create_droplet_snapshot: compensated, fail: failed], zero leftover on DO.

Test-harness note: smoke tests create sagas via Provisioning.create_saga
(no Oban enqueue) so a single manual Runner.perform/1 owns execution —
start_saga/1 enqueues an Oban job, and running both racing the same saga
corrupts the step ledger. Production only ever runs via Oban.

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

lib/arcadia_cloud/provisioning.ex

@@ -56,6 +56,23 @@ defmodule ArcadiaCloud.Provisioning do
     |> Repo.insert()
   end
 
+  @doc """
+  Starts a snapshot saga for a droplet. `droplet_provider_id` is the DO
+  numeric droplet id (string). Optional `:snapshot_label` and
+  `:triggered_by`.
+  """
+  def snapshot_droplet(droplet_provider_id, opts \\ []) do
+    start_saga(%{
+      kind: "provision",
+      step_modules: [ArcadiaCloud.Provisioning.Steps.CreateDropletSnapshot],
+      inputs: %{
+        droplet_provider_id: to_string(droplet_provider_id),
+        snapshot_label: opts[:snapshot_label]
+      },
+      triggered_by: opts[:triggered_by] || "manual"
+    })
+  end
+
   def get_saga(id), do: Repo.get(SagaRun, id)
   def get_saga!(id), do: Repo.get!(SagaRun, id)