Phase 3: tenant invoice rollup

Month-end engine — turns a period of metered usage into tenant invoices
(revenue side). Distinct from cloud_invoices, which are DO's bills to
Sky AI (COGS).

tenant_invoices — one per (tenant, period). subtotal/tax/total cents,
status draft/issued/paid/void. unique (tenant_id, period_start).
tenant_invoice_lines — kind plan_base/addon/overage/tax, tagged with
deployment_id (NULL for tenant-level lines like GST) + resource_kind,
so the cost-vs-revenue dashboard can group by deployment and by kind.

ArcadiaCloud.Invoicing.roll_up_period/3:
- groups active subscriptions by tenant
- one tenant_invoice per tenant; per subscription, runs the quote engine
  with the deployment's ACTUAL metered usage (Metering.usage_for_period)
  and persists the recurring + overage lines tagged with the deployment
- appends a tenant-level GST line (AU 10%, per project_skyai_australia)
- idempotent on (tenant_id, period_start); re-run skips unless force:true

Because the same quote engine serves provisioning-time projection and
month-end invoicing, a tenant's quoted price and invoiced price are
computed identically.

InvoiceRollupWorker — Oban cron, 1st of month 03:00 UTC, invoices the
month just ended.

API (platform_admin sees all; tenants scoped to own):
- GET /api/v1/invoices       — tenant invoice list
- GET /api/v1/invoices/:id   — invoice with lines

Also: SubscriptionAddon now preloads its :addon so quote/invoice lines
read "Addon: storage_50gb" rather than the addon UUID.

Smoke verified: pilot deployment on Studio + storage_50gb, 3 droplets
metered across all 30 days of April (2160 droplet_hours vs 1488
included) — rollup produced an invoice with plan_base $50 + addon $7.50
+ droplet_hours overage $6.72 (672h x 1c) = $64.22 subtotal, GST $6.42,
total $70.64. Re-run without force correctly skipped.

NOT in this chunk: pushing tenant invoices to skyai-finance as AR —
that needs an income-side endpoint on skyai-finance (the phase-1 push
endpoint creates vendor expense invoices, wrong direction). Deferred to
its own chunk.

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

lib/arcadia_cloud/quoting.ex

@@ -102,8 +102,17 @@ defmodule ArcadiaCloud.Quoting do
 
   defp addon_fields(%Addon{} = a), do: {a.code, a.resource_kind, a.qty, a.price_cents}
 
-  defp addon_fields(%SubscriptionAddon{} = sa),
-    do: {sa.addon_id, sa.resource_kind, sa.qty, sa.price_cents}
+  defp addon_fields(%SubscriptionAddon{} = sa) do
+    # prefer the snapshotted addon's code for a readable line; fall back
+    # to the id if the :addon association wasn't preloaded.
+    code =
+      case sa.addon do
+        %Addon{code: code} -> code
+        _ -> sa.addon_id
+      end
+
+    {code, sa.resource_kind, sa.qty, sa.price_cents}
+  end
 
   # ---- overage --------------------------------------------------------------