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/projects_worker.ex

@@ -64,17 +64,35 @@ defmodule ArcadiaCloud.Sync.ProjectsWorker do
     end)
   end
 
+  # Kinds with a dedicated sync worker — ProjectsWorker only updates attribution
+  # for these, never inserts (the worker is the source of truth for richer attrs).
+  @dedicated_kinds ~w(droplet volume snapshot floating_ip firewall load_balancer dns_zone)
+
   defp attribute_urns(urns, %{id: project_id} = local) do
     tenant_id = tenant_id_for(local)
 
     Enum.each(urns, fn %{"urn" => urn} ->
       case parse_urn(urn) do
-        {kind, provider_id} -> update_resource_attribution(kind, provider_id, project_id, tenant_id)
-        _ -> :skip
+        {kind, provider_id} ->
+          attribute_or_discover(kind, provider_id, project_id, tenant_id)
+
+        _ ->
+          :skip
       end
     end)
   end
 
+  # For kinds with a dedicated worker, just update attribution. For everything
+  # else (spaces_bucket, managed_db, k8s_cluster, etc.) insert a minimal
+  # placeholder so the resource shows up in inventory + cost matching.
+  defp attribute_or_discover(kind, provider_id, project_id, tenant_id) do
+    if kind in @dedicated_kinds do
+      update_resource_attribution(kind, provider_id, project_id, tenant_id)
+    else
+      ensure_via_urn(kind, provider_id, project_id, tenant_id)
+    end
+  end
+
   defp update_resource_attribution(kind, provider_id, project_id, tenant_id) do
     from(r in CloudResource,
       where:
@@ -84,6 +102,26 @@ defmodule ArcadiaCloud.Sync.ProjectsWorker do
     |> Repo.update_all(set: [cloud_project_id: project_id, tenant_id: tenant_id])
   end
 
+  defp ensure_via_urn(kind, provider_id, project_id, tenant_id) do
+    now = DateTime.utc_now() |> DateTime.truncate(:second)
+
+    Cloud.upsert_resource(
+      %{
+        provider: "digitalocean",
+        provider_id: provider_id,
+        kind: kind,
+        name: provider_id,
+        status: "active",
+        cloud_project_id: project_id,
+        tenant_id: tenant_id,
+        attrs: %{discovered_via: "urn_membership"},
+        first_seen_at: now,
+        last_seen_at: now
+      },
+      source: "projects_urn"
+    )
+  end
+
   # "do:droplet:567897199" → {"droplet", "567897199"}
   defp parse_urn("do:" <> rest) do
     case String.split(rest, ":", parts: 2) do
@@ -101,6 +139,8 @@ defmodule ArcadiaCloud.Sync.ProjectsWorker do
   defp normalize_kind("floatingip"), do: "floating_ip"
   defp normalize_kind("loadbalancer"), do: "load_balancer"
   defp normalize_kind("dbaas"), do: "managed_db"
+  defp normalize_kind("volumesnapshot"), do: "snapshot"
+  defp normalize_kind("image"), do: "snapshot"
   defp normalize_kind(other), do: other
 
   defp tenant_id_for(%{name: "tenant-" <> tenant_uuid}), do: tenant_uuid