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/digital_ocean/client.ex

@@ -21,6 +21,52 @@ defmodule ArcadiaCloud.DigitalOcean.Client do
   def list_volumes(opts \\ []), do: list_paginated("/volumes", "volumes", opts)
   def list_floating_ips(opts \\ []), do: list_paginated("/floating_ips", "floating_ips", opts)
 
+  # ---- billing --------------------------------------------------------------
+
+  def get_balance(opts \\ []) do
+    request(:get, "/customers/my/balance", purpose: opts[:purpose] || "billing")
+  end
+
+  def list_billing_history(opts \\ []) do
+    list_paginated("/customers/my/billing_history", "billing_history",
+      Keyword.put(opts, :purpose, opts[:purpose] || "billing"))
+  end
+
+  def get_invoice_summary(invoice_uuid, opts \\ []) do
+    request(:get, "/customers/my/invoices/#{invoice_uuid}/summary",
+      purpose: opts[:purpose] || "billing")
+  end
+
+  @doc """
+  Fetch the CSV body for an invoice. Returns {:ok, csv_string} | {:error, _}.
+  """
+  def fetch_invoice_csv(invoice_uuid, opts \\ []) do
+    purpose = opts[:purpose] || "billing"
+
+    with {:ok, token} <- Tokens.fetch(purpose) do
+      case Req.request(
+             method: :get,
+             url: @base <> "/customers/my/invoices/#{invoice_uuid}/csv",
+             headers: [
+               {"authorization", "Bearer " <> token},
+               {"accept", "text/csv"}
+             ],
+             retry: :transient,
+             max_retries: 3,
+             decode_body: false
+           ) do
+        {:ok, %Req.Response{status: 200, body: body}} when is_binary(body) ->
+          {:ok, body}
+
+        {:ok, %Req.Response{status: status, body: body}} ->
+          {:error, {:http, status, body}}
+
+        {:error, e} ->
+          {:error, {:transport, e}}
+      end
+    end
+  end
+
   def create_project(name, purpose, description \\ "", opts \\ []) do
     body = %{
       name: name,