Phase 3: usage metering

usage_records — one metered observation per (deployment, resource,
period, granularity). resource_kind is the BILLING kind (droplet_hours,
spaces_gb_month, ...) so it joins straight to plan_items. sub_attribution
jsonb column present from day one for forward-compat (sub-billing) but
never written in phase 3.

metering_config — per-billing-kind granularity (daily default; flip to
hourly when fine-grained billing is wanted) + retention_days.

ArcadiaCloud.Metering:
- meter_day/1 — walks every deployment-attributed resource, maps its
  inventory kind to a billing kind, records that day's usage. Idempotent
  via the unique (deployment, resource, period, granularity) index.
- usage_for_period/3 — SUM(qty) GROUP BY billing kind over a date range,
  returns the %{kind => qty} map the quote engine takes as :usage.

Uniform accrual model so everything downstream is just SUM(qty):
- droplet      -> droplet_hours, 24/day when active (0 when off)
- spaces_bucket / snapshot / droplet_backup -> *_gb_month, size/days so
  the month sums to GB-months
- dns_zone     -> dns_zones, 1/days so the month sums to the zone count

MeteringWorker — Oban cron 01:10 UTC daily, meters the previous
complete day.

Smoke verified: a pilot deployment with 2 droplets + 2 DNS zones + 3
snapshots attributed; 10 days metered -> 70 usage records; aggregation
gave droplet_hours 480 (2x24x10), dns_zones 0.65 (2x10/31),
snapshot_gb_month 15.48; fed into the quote engine against Studio — all
within allowance so $50 base, no overage (correct for a light partial
month).

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

config/config.exs

@@ -65,6 +65,8 @@ config :arcadia_cloud, Oban,
        {"41 * * * *", ArcadiaCloud.Sync.BackupsWorker},
        # Drift sweep — offset past the :15 resource syncs so it sees fresh data
        {"20 * * * *", ArcadiaCloud.Sync.DriftDetectionWorker},
+       # Metering — daily at 01:10 UTC, meters the previous complete day
+       {"10 1 * * *", ArcadiaCloud.Sync.MeteringWorker},
        # Billing: hourly balance, daily invoice discovery
        {"7 * * * *", ArcadiaCloud.Sync.BalanceWorker},
        {"23 2 * * *", ArcadiaCloud.Sync.BillingHistoryWorker}