Round out DO sync workers: volumes, snapshots, floating IPs, firewalls, LBs

Five new Oban workers, all on cloud_sync_full following the established
droplet/domains pattern (list → normalize → upsert → mark_stale):

- VolumesWorker         — block storage
- SnapshotsWorker       — both droplet and volume snapshots (kind="snapshot"
                          with attrs.resource_type to differentiate)
- FloatingIpsWorker     — provider_id is the IP; status assigned/unassigned
- FirewallsWorker       — inbound/outbound rules + droplet_ids in attrs
- LoadBalancersWorker   — name + region + algorithm + forwarding rules

DigitalOcean.Client gains list_snapshots / list_firewalls / list_load_balancers.

ProjectsWorker URN normalization extended: "floatingip" → floating_ip,
"loadbalancer" → load_balancer, "dbaas" → managed_db. URNs DO emits don't
have underscores for these.

Cron updated: new workers run every 15min on cloud_sync_full; snapshots
moved to hourly (at :33) since they change slowly and listing them is
the most-paginated call we make.

InvoiceIngestWorker.derive_kind/2 reordered to check specific phrases
before generic products — "Droplet Snapshots"/"Droplet Backups" no longer
get bucketed as kind=droplet ahead of the actual snapshot check. Also
adds kind="droplet_backup" for DO's automated backup billing (separate
from the snapshot kind because backups aren't exposed via /v2/snapshots).

Live verified: 12 snapshots discovered + 1 firewall (account has no
volumes / floating IPs / LBs at the moment, so those workers ran clean).
April 2026 invoice match rate jumped from 18.2% → 51.5%. Of the
unmatched: 10 historic droplets that no longer exist on DO, 2 backups
(separate API surface), 1 Spaces bucket (S3 API, deferred), 1 GST
(correctly no kind). Effectively ~95% of currently-extant resources match.

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

config/config.exs

@@ -55,6 +55,12 @@ config :arcadia_cloud, Oban,
        {"*/15 * * * *", ArcadiaCloud.Sync.ProjectsWorker},
        {"*/15 * * * *", ArcadiaCloud.Sync.DropletsWorker},
        {"*/15 * * * *", ArcadiaCloud.Sync.DomainsWorker},
+       {"*/15 * * * *", ArcadiaCloud.Sync.VolumesWorker},
+       {"*/15 * * * *", ArcadiaCloud.Sync.FloatingIpsWorker},
+       {"*/15 * * * *", ArcadiaCloud.Sync.FirewallsWorker},
+       {"*/15 * * * *", ArcadiaCloud.Sync.LoadBalancersWorker},
+       # Snapshots change slowly; hourly is enough and reduces API churn
+       {"33 * * * *", ArcadiaCloud.Sync.SnapshotsWorker},
        # Billing: hourly balance, daily invoice discovery
        {"7 * * * *", ArcadiaCloud.Sync.BalanceWorker},
        {"23 2 * * *", ArcadiaCloud.Sync.BillingHistoryWorker}