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>

config/config.exs

@@ -61,6 +61,8 @@ config :arcadia_cloud, Oban,
        {"*/15 * * * *", ArcadiaCloud.Sync.LoadBalancersWorker},
        # Snapshots change slowly; hourly is enough and reduces API churn
        {"33 * * * *", ArcadiaCloud.Sync.SnapshotsWorker},
+       # Backups also slow-moving; hourly per-droplet walk
+       {"41 * * * *", ArcadiaCloud.Sync.BackupsWorker},
        # Billing: hourly balance, daily invoice discovery
        {"7 * * * *", ArcadiaCloud.Sync.BalanceWorker},
        {"23 2 * * *", ArcadiaCloud.Sync.BillingHistoryWorker}