Phase 1 cost ingestion: balance + invoices + CSV parse + resource match

Three new schemas:
- cloud_balance_snapshots — hourly MTD balance/usage poll for live-accrual.
- cloud_invoices — header per provider invoice, with ingest status flags.
- cloud_cost_lines — per-line-item COGS, FK to cloud_resources where matched.

Three new Oban workers (queue: cloud_billing):
- BalanceWorker (hourly) records a snapshot.
- BillingHistoryWorker (daily) discovers invoices via /v2/customers/my/
  billing_history, upserts headers, enqueues an InvoiceIngestWorker for
  each not-yet-ingested invoice.
- InvoiceIngestWorker (per-invoice) fetches /invoices/:uuid/csv, parses
  with NimbleCSV (header-keyed so column order shifts don't break us),
  replaces the invoice's line set, then matches lines to cloud_resources
  by (kind, name) — case-insensitive, name extracted from "name (size)"
  description format.

DigitalOcean.Client gains get_balance / list_billing_history /
get_invoice_summary / fetch_invoice_csv. The CSV endpoint returns text/csv
so we bypass Req's body decoder.

Cron additions: BalanceWorker hourly at :07, BillingHistoryWorker daily
at 02:23.

API:
- GET /api/v1/billing/balance — latest snapshot, platform_admin only.
- GET /api/v1/billing/cost-lines?period=YYYY-MM-DD&kind&limit — per-line
  COGS, platform_admin only.

Live smoke against real DO billing API surfaced and fixed three CSV-format
gotchas: column headers use underscores not spaces (group_description,
project_name), USD column has $ prefix, dates use "YYYY-MM-DD HH:MM:SS
+0000" format (space separator + RFC822 offset).

Verified: 137 historical invoices discovered going back to 2014;
April 2026 invoice (33 lines, $86.92 total) ingested with 6/33 lines
matched to current cloud_resources. Unmatched lines are correctly
historic droplets, Spaces buckets (not yet synced), and GST.

NimbleCSV ~> 1.2 added as a dep.

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

lib/arcadia_cloud/sync/balance_worker.ex

@@ -0,0 +1,47 @@
+defmodule ArcadiaCloud.Sync.BalanceWorker do
+  @moduledoc """
+  Hourly poll of `/v2/customers/my/balance`. Records a snapshot row so
+  the cost dashboard can show MTD usage in real time.
+  """
+
+  use Oban.Worker, queue: :cloud_billing, max_attempts: 3
+
+  alias ArcadiaCloud.Billing
+  alias ArcadiaCloud.DigitalOcean.Client
+
+  @impl Oban.Worker
+  def perform(_job) do
+    with {:ok, body} <- Client.get_balance() do
+      Billing.record_balance(%{
+        provider: "digitalocean",
+        month_to_date_balance_cents: dollars_to_cents(body["month_to_date_balance"]),
+        account_balance_cents: dollars_to_cents(body["account_balance"]),
+        month_to_date_usage_cents: dollars_to_cents(body["month_to_date_usage"]),
+        generated_at: parse_iso(body["generated_at"]),
+        raw: body
+      })
+
+      :ok
+    end
+  end
+
+  defp dollars_to_cents(nil), do: nil
+
+  defp dollars_to_cents(value) when is_binary(value) do
+    case Float.parse(value) do
+      {float, _} -> round(float * 100)
+      :error -> nil
+    end
+  end
+
+  defp dollars_to_cents(value) when is_number(value), do: round(value * 100)
+
+  defp parse_iso(nil), do: DateTime.utc_now() |> DateTime.truncate(:second)
+
+  defp parse_iso(str) when is_binary(str) do
+    case DateTime.from_iso8601(str) do
+      {:ok, dt, _} -> DateTime.truncate(dt, :second)
+      _ -> DateTime.utc_now() |> DateTime.truncate(:second)
+    end
+  end
+end