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/billing.ex

@@ -129,8 +129,24 @@ defmodule ArcadiaCloud.Billing do
     end)
   end
 
-  defp find_resource(%CloudCostLine{kind: kind, description: desc}) when is_binary(desc) do
-    name_lower = desc |> extract_name() |> String.downcase()
+  # Droplet backups are billed as a per-droplet subscription. The CSV line
+  # description carries the droplet name ("git.sky-ai.com (Weekly Backup
+  # Services)") so we match the line to the parent droplet, not a specific
+  # backup snapshot.
+  defp find_resource(%CloudCostLine{kind: "droplet_backup", description: desc})
+       when is_binary(desc) do
+    find_by_kind_and_name("droplet", desc)
+  end
+
+  defp find_resource(%CloudCostLine{kind: kind, description: desc})
+       when is_binary(desc) and is_binary(kind) do
+    find_by_kind_and_name(kind, desc)
+  end
+
+  defp find_resource(_), do: nil
+
+  defp find_by_kind_and_name(kind, description) do
+    name_lower = description |> extract_name() |> String.downcase()
 
     from(r in CloudResource,
       where:
@@ -141,8 +157,6 @@ defmodule ArcadiaCloud.Billing do
     |> Repo.one()
   end
 
-  defp find_resource(_), do: nil
-
   # DO CSV description is often "name (size_slug)" or "name-1234 (region) NGB Snapshot".
   # Strip everything after the first " (" — best-effort name extraction.
   defp extract_name(desc) do