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/invoicing.ex

@@ -77,7 +77,12 @@ defmodule ArcadiaCloud.Invoicing do
         })
         |> Repo.insert()
 
-      lines = Enum.flat_map(subscriptions, &subscription_lines(&1, period_start, period_end))
+      # Infra lines from the quote engine + LLM-token lines from metered
+      # gateway usage (arcadia-llm-gateway pushes priced usage events).
+      lines =
+        Enum.flat_map(subscriptions, &subscription_lines(&1, period_start, period_end)) ++
+          ArcadiaCloud.LlmUsage.period_lines(tenant_id, period_start, period_end)
+
       Enum.each(lines, &insert_line(invoice.id, &1))
 
       subtotal = Enum.reduce(lines, 0, &(&1.amount_cents + &2))