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>

lib/arcadia_cloud/metering/metering_config.ex

@@ -0,0 +1,25 @@
+defmodule ArcadiaCloud.Metering.MeteringConfig do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  @granularities ~w(daily hourly)
+
+  schema "metering_config" do
+    field :resource_kind, :string
+    field :granularity, :string, default: "daily"
+    field :retention_days, :integer, default: 400
+
+    timestamps(type: :utc_datetime)
+  end
+
+  def changeset(config, attrs) do
+    config
+    |> cast(attrs, [:resource_kind, :granularity, :retention_days])
+    |> validate_required([:resource_kind, :granularity])
+    |> validate_inclusion(:granularity, @granularities)
+    |> unique_constraint(:resource_kind)
+  end
+end