Receive llm_usage_recorded events from arcadia-llm-gateway

POST /api/v1/integrations/llm-usage stores priced LLM usage events
(idempotent on gateway_request_id) in llm_usage_events. The gateway is
the LLM-pricing authority — arcadia-cloud trusts the charge it sends
rather than re-pricing.

The monthly invoice rollup now appends an llm_usage line per deployment
alongside the infra quote lines; the exact decimal charges are summed
and rounded to cents once. Closes the gateway→cloud billing loop.

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

lib/arcadia_cloud/billing/llm_usage_event.ex

@@ -0,0 +1,43 @@
+defmodule ArcadiaCloud.Billing.LlmUsageEvent do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "llm_usage_events" do
+    field :gateway_request_id, :string
+    field :tenant_id, :string
+    field :deployment_id, :string
+
+    field :provider, :string
+    field :model, :string
+    field :request_kind, :string
+
+    field :input_tokens, :integer, default: 0
+    field :output_tokens, :integer, default: 0
+    field :cached_input_tokens, :integer, default: 0
+    field :total_tokens, :integer, default: 0
+
+    field :upstream_cost, :decimal
+    field :customer_charge, :decimal
+    field :customer_charge_cents, :integer, default: 0
+    field :markup_mode, :string
+
+    field :occurred_at, :utc_datetime_usec
+
+    timestamps(type: :utc_datetime, updated_at: false)
+  end
+
+  @required ~w(gateway_request_id tenant_id occurred_at)a
+  @optional ~w(deployment_id provider model request_kind input_tokens
+               output_tokens cached_input_tokens total_tokens upstream_cost
+               customer_charge customer_charge_cents markup_mode)a
+
+  def changeset(event, attrs) do
+    event
+    |> cast(attrs, @required ++ @optional)
+    |> validate_required(@required)
+    |> unique_constraint(:gateway_request_id)
+  end
+end