Fix operator role gate: platform-admin (hyphen), not platform_admin

arcadia-app issues the role slug "platform-admin" (hyphen) — confirmed
from a live arcadia-dev JWT (roles: ["admin","platform-admin"]). Every
authorization check here tested for "platform_admin" (underscore), so
real operator tokens got 403 on billing / dashboard / drift and an
empty tenant-scoped result on inventory.

The smoke tests missed it because Guardian.mint_dev_token hardcoded the
underscore form — fixed there too, so the dev helper now matches what
arcadia-app actually emits.

Replaced the string literal "platform_admin" -> "platform-admin" in all
six controllers + guardian.ex. The platform_admin?/1 function names keep
underscores (Elixir identifiers can't contain hyphens) — only the role
string changed.

Verified: with a platform-admin token, /inventory, /billing/balance,
/dashboard/margin and /drift all return 200.

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

lib/arcadia_cloud_web/controllers/dashboard_controller.ex

@@ -31,7 +31,7 @@ defmodule ArcadiaCloudWeb.DashboardController do
   defp require_platform_admin(conn) do
     identity = conn.assigns.current_identity
 
-    if is_list(identity.roles) and "platform_admin" in identity.roles do
+    if is_list(identity.roles) and "platform-admin" in identity.roles do
       :ok
     else
       conn |> put_status(:forbidden) |> json(%{error: "platform_admin_required"}) |> halt()