Phase 3: quote engine

ArcadiaCloud.Quoting.quote/3 — turns a plan version + addons + usage into
itemized pricing. The same function serves both provisioning-time
projection (with projected usage) and month-end invoicing (with actual
metered usage), so the two can never disagree on the math.

A quote has:
- recurring     — plan base + addon lines (fixed monthly)
- first_invoice — recurring prorated to a mid-month start date
                  (fraction = days_remaining / days_in_month)
- overage       — usage beyond plan+addon allowance, billed in arrears
- all_in_monthly_cents

Overage computation: per plan_item, effective allowance =
included_qty + summed addon qty for the same resource_kind. Overage =
max(0, used - allowance), clamped by hard_cap_qty. overage_price_cents
is per overage_unit; a @unit_factor map converts base-unit usage to
overage units (e.g. 1k_tokens divides token counts by 1000).

Lines are uniform maps (kind/resource_kind/description/qty/unit/
unit_price_cents/amount_cents/meta) — the invoice rollup chunk will
persist them near-verbatim as invoice_lines.

addon_fields/1 accepts either a catalog Addon (projection from the
catalog) or a SubscriptionAddon (actual, using snapshotted price/qty).

API: POST /api/v1/quote { plan_code, addons[], projected_usage{},
starts_on } — tenant-facing pre-provisioning price preview.

Smoke verified against seeded plans:
- Studio bare: $50/mo, no overage.
- Studio + storage_50gb, mid-month start 2026-05-20, projected usage:
  recurring $57.50; first invoice prorated 12/31 -> $22.25; overage
  LLM 500k-token $5.00 + Spaces 50GB $1.00 (the addon correctly lifted
  the Spaces allowance 100->150 GB); droplet_hours exactly at allowance
  produced no line; all-in $63.50/mo.

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

lib/arcadia_cloud_web/controllers/quote_controller.ex

@@ -0,0 +1,52 @@
+defmodule ArcadiaCloudWeb.QuoteController do
+  @moduledoc """
+  Pricing quotes. A tenant hits this before provisioning to see what a
+  plan + addons would cost, optionally with projected usage to preview
+  likely overage.
+
+  Body:
+    plan_code        — required
+    addons           — optional list of addon codes
+    projected_usage  — optional map resource_kind => qty
+    starts_on        — optional ISO date; if set, returns prorated
+                       first-invoice figures
+  """
+
+  use ArcadiaCloudWeb, :controller
+
+  alias ArcadiaCloud.{Catalog, Quoting}
+
+  def create(conn, params) do
+    plan = params["plan_code"] && Catalog.get_plan_by_code(params["plan_code"])
+    version = plan && Catalog.active_version(plan)
+
+    cond do
+      is_nil(version) ->
+        conn |> put_status(:not_found) |> json(%{error: "plan_or_version_not_found"})
+
+      true ->
+        addons = Catalog.get_addons(params["addons"] || [])
+
+        opts =
+          []
+          |> put_opt(:usage, params["projected_usage"])
+          |> put_opt(:starts_on, parse_date(params["starts_on"]))
+
+        quote = Quoting.quote(version, addons, opts)
+        json(conn, %{quote: quote, plan_code: plan.code})
+    end
+  end
+
+  defp put_opt(opts, _key, nil), do: opts
+  defp put_opt(opts, key, value), do: Keyword.put(opts, key, value)
+
+  defp parse_date(nil), do: nil
+  defp parse_date(""), do: nil
+
+  defp parse_date(s) do
+    case Date.from_iso8601(s) do
+      {:ok, d} -> d
+      _ -> nil
+    end
+  end
+end