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>

priv/repo/migrations/20260520160000_create_metering.exs

@@ -0,0 +1,46 @@
+defmodule ArcadiaCloud.Repo.Migrations.CreateMetering do
+  use Ecto.Migration
+
+  def change do
+    # Per-resource-kind metering granularity. Daily by default; flip a
+    # kind to hourly when fine-grained billing is wanted.
+    create table(:metering_config, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :resource_kind, :string, null: false
+      add :granularity, :string, null: false, default: "daily"
+      add :retention_days, :integer, null: false, default: 400
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:metering_config, [:resource_kind])
+
+    # One metered observation per (deployment, resource, period, granularity).
+    # resource_kind here is the BILLING kind (droplet_hours, spaces_gb_month,
+    # ...) so it joins straight to plan_items.resource_kind.
+    create table(:usage_records, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :deployment_id,
+        references(:cloud_deployments, type: :binary_id, on_delete: :delete_all),
+        null: false
+      add :resource_id, references(:cloud_resources, type: :binary_id, on_delete: :nilify_all)
+      add :resource_kind, :string, null: false
+      add :period_start, :utc_datetime, null: false
+      add :granularity, :string, null: false, default: "daily"
+      add :qty, :decimal, null: false, default: 0
+      # forward-compat: tenants opting into sub-billing later can attribute
+      # usage to their own end-users without a schema migration. Never
+      # written in phase 3.
+      add :sub_attribution, :map
+
+      timestamps(type: :utc_datetime, updated_at: false)
+    end
+
+    create unique_index(:usage_records, [:deployment_id, :resource_id, :period_start, :granularity],
+             name: :usage_records_unique
+           )
+
+    create index(:usage_records, [:deployment_id, :period_start])
+    create index(:usage_records, [:resource_kind])
+  end
+end