Phase 0 scaffold: arcadia-cloud Phoenix service

API-only Phoenix 1.8 project for cloud-ops, inventory, billing, and
provisioning sagas. Validates arcadia JWTs via shared Guardian secret
(verify-only; arcadia-app remains the issuer).

Deps beyond default Phoenix: guardian, cors_plug, oban, req.
Postgres on local port 5433 per arcadia stack convention.
Endpoint runs on :4005.

Endpoints:
- GET /api/health         — public, returns service identifier
- GET /api/v1/inventory   — auth-gated, returns empty list (phase 0 stub)

Oban configured with the queues phase 1+ will need:
provisioning / cloud_sync_fast|full|slow / cloud_billing / metering.

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

priv/repo/migrations/.formatter.exs

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+[
+  import_deps: [:ecto_sql],
+  inputs: ["*.exs"]
+]

priv/repo/migrations/20260519000000_add_oban_jobs.exs

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+defmodule ArcadiaCloud.Repo.Migrations.AddObanJobs do
+  use Ecto.Migration
+
+  def up do
+    Oban.Migration.up(version: 12)
+  end
+
+  def down do
+    Oban.Migration.down(version: 1)
+  end
+end

priv/repo/seeds.exs

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+# Script for populating the database. You can run it as:
+#
+#     mix run priv/repo/seeds.exs
+#
+# Inside the script, you can read and write to any of your
+# repositories directly:
+#
+#     ArcadiaCloud.Repo.insert!(%ArcadiaCloud.SomeSchema{})
+#
+# We recommend using the bang functions (`insert!`, `update!`
+# and so on) as they will fail if something goes wrong.