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/billing/cloud_balance_snapshot.ex

@@ -0,0 +1,27 @@
+defmodule ArcadiaCloud.Billing.CloudBalanceSnapshot do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "cloud_balance_snapshots" do
+    field :provider, :string
+    field :month_to_date_balance_cents, :integer
+    field :account_balance_cents, :integer
+    field :month_to_date_usage_cents, :integer
+    field :generated_at, :utc_datetime
+    field :raw, :map
+
+    timestamps(type: :utc_datetime, updated_at: false)
+  end
+
+  @required ~w(provider generated_at)a
+  @optional ~w(month_to_date_balance_cents account_balance_cents month_to_date_usage_cents raw)a
+
+  def changeset(snap, attrs) do
+    snap
+    |> cast(attrs, @required ++ @optional)
+    |> validate_required(@required)
+  end
+end

lib/arcadia_cloud/billing/cloud_cost_line.ex

@@ -0,0 +1,38 @@
+defmodule ArcadiaCloud.Billing.CloudCostLine do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "cloud_cost_lines" do
+    field :invoice_period, :date
+    field :kind, :string
+    field :description, :string
+    field :qty, :decimal
+    field :unit, :string
+    field :unit_cost_cents, :integer
+    field :amount_cents, :integer
+    field :start_at, :utc_datetime
+    field :end_at, :utc_datetime
+    field :project_name, :string
+    field :category, :string
+    field :matched_at, :utc_datetime
+    field :raw, :map
+
+    belongs_to :invoice, ArcadiaCloud.Billing.CloudInvoice
+    belongs_to :resource, ArcadiaCloud.Cloud.CloudResource
+
+    timestamps(type: :utc_datetime, updated_at: false)
+  end
+
+  @required ~w(invoice_id invoice_period amount_cents)a
+  @optional ~w(resource_id kind description qty unit unit_cost_cents start_at end_at
+               project_name category matched_at raw)a
+
+  def changeset(line, attrs) do
+    line
+    |> cast(attrs, @required ++ @optional)
+    |> validate_required(@required)
+  end
+end

lib/arcadia_cloud/billing/cloud_invoice.ex

@@ -0,0 +1,31 @@
+defmodule ArcadiaCloud.Billing.CloudInvoice do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "cloud_invoices" do
+    field :provider, :string
+    field :provider_invoice_id, :string
+    field :invoice_period, :date
+    field :amount_cents, :integer
+    field :status, :string, default: "open"
+    field :issued_at, :utc_datetime
+    field :csv_fetched_at, :utc_datetime
+    field :lines_ingested_at, :utc_datetime
+    field :raw, :map
+
+    timestamps(type: :utc_datetime)
+  end
+
+  @required ~w(provider provider_invoice_id invoice_period)a
+  @optional ~w(amount_cents status issued_at csv_fetched_at lines_ingested_at raw)a
+
+  def changeset(invoice, attrs) do
+    invoice
+    |> cast(attrs, @required ++ @optional)
+    |> validate_required(@required)
+    |> unique_constraint([:provider, :provider_invoice_id])
+  end
+end