Close inventory gaps: Spaces buckets (URN-discover), droplet backups, snapshot URN aliases

Two patterns added:

1. ProjectsWorker now does URN-discover for kinds without a dedicated
   sync worker (spaces_bucket, managed_db, k8s_cluster, etc.). For these,
   it inserts a minimal placeholder row when the URN points to something
   not yet in inventory. Kinds with dedicated workers (droplet, snapshot,
   volume, etc.) still get attribution-only — the worker is source of
   truth for richer attrs. Implemented by splitting attribute_or_discover/4
   on a @dedicated_kinds whitelist.

2. New BackupsWorker pulls /v2/droplets/:id/backups for each active
   droplet. DO automated backups aren't in /v2/snapshots; they live per
   droplet. Cron: hourly at :41. Kind="droplet_backup".

URN normalization extended for two more aliases DO emits:
  "volumesnapshot" → snapshot   (was creating a duplicate row)
  "image"          → snapshot   (DO droplet snapshots show as do:image:id)

Billing.find_resource/1 gets a kind-specific clause for droplet_backup
that matches to the parent droplet by name, since invoice lines for
backups read "<droplet-name> (Weekly Backup Services)" — the line is a
per-droplet subscription, not a per-backup-snapshot fee.

Live verified on the same April 2026 invoice:
- 6 Spaces buckets discovered via URN (account has 6, only 1 visible in
  the invoice as the $5 subscription line — that's account-level so it
  can't tie to a specific bucket, expected).
- 4 droplet backups discovered via BackupsWorker; the git.sky-ai.com
  backup line now matches (repo.sky-ai.com backup line can't match — that
  droplet was destroyed).
- Of 16 unmatched lines: 11 are destroyed historic resources, 1 is GST,
  1 is the account-level Spaces subscription, 3 are likely tiny snapshot
  name variances. Effectively ~100% of currently-existing billable
  resources match.

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

lib/arcadia_cloud/sync/backups_worker.ex

@@ -0,0 +1,84 @@
+defmodule ArcadiaCloud.Sync.BackupsWorker do
+  @moduledoc """
+  Sync of DO automated droplet backups.
+
+  Backups are not exposed via /v2/snapshots — they live under each
+  droplet at /v2/droplets/:id/backups. We iterate every active droplet
+  in inventory and pull its backups, normalizing them as
+  kind="droplet_backup" with the parent droplet_id in attrs.
+  """
+
+  use Oban.Worker, queue: :cloud_sync_full, max_attempts: 3
+
+  import Ecto.Query
+
+  alias ArcadiaCloud.Cloud
+  alias ArcadiaCloud.Cloud.CloudResource
+  alias ArcadiaCloud.DigitalOcean.Client
+  alias ArcadiaCloud.Repo
+
+  @kind "droplet_backup"
+  @provider "digitalocean"
+
+  @impl Oban.Worker
+  def perform(_job) do
+    now = DateTime.utc_now() |> DateTime.truncate(:second)
+    droplets = list_active_droplets()
+
+    Enum.each(droplets, fn d ->
+      case Client.list_droplet_backups(d.provider_id) do
+        {:ok, backups} ->
+          Enum.each(backups, fn b ->
+            Cloud.upsert_resource(normalize(b, d, now))
+          end)
+
+        {:error, _} ->
+          # Soft-fail per droplet; mark_stale below handles disappearances.
+          :skip
+      end
+    end)
+
+    Cloud.mark_stale(@kind, now)
+    :ok
+  end
+
+  defp list_active_droplets do
+    from(r in CloudResource,
+      where:
+        r.provider == ^@provider and r.kind == "droplet" and is_nil(r.deleted_at) and
+          r.status != "archived",
+      select: %{id: r.id, provider_id: r.provider_id, cloud_project_id: r.cloud_project_id,
+               tenant_id: r.tenant_id}
+    )
+    |> Repo.all()
+  end
+
+  defp normalize(b, droplet, now) do
+    region =
+      case b["regions"] do
+        [first | _] when is_binary(first) -> first
+        _ -> nil
+      end
+
+    %{
+      provider: @provider,
+      provider_id: to_string(b["id"]),
+      kind: @kind,
+      name: b["name"] || "backup-#{b["id"]}",
+      region: region,
+      status: "active",
+      tags: [],
+      cloud_project_id: droplet.cloud_project_id,
+      tenant_id: droplet.tenant_id,
+      attrs: %{
+        droplet_id: droplet.provider_id,
+        size_gigabytes: b["size_gigabytes"],
+        min_disk_size: b["min_disk_size"],
+        regions: b["regions"],
+        do_created_at: b["created_at"]
+      },
+      first_seen_at: now,
+      last_seen_at: now
+    }
+  end
+end