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>

README.md

@@ -0,0 +1,18 @@
+# ArcadiaCloud
+
+To start your Phoenix server:
+
+* Run `mix setup` to install and setup dependencies
+* Start Phoenix endpoint with `mix phx.server` or inside IEx with `iex -S mix phx.server`
+
+Now you can visit [`localhost:4000`](http://localhost:4000) from your browser.
+
+Ready to run in production? Please [check our deployment guides](https://hexdocs.pm/phoenix/deployment.html).
+
+## Learn more
+
+* Official website: https://www.phoenixframework.org/
+* Guides: https://hexdocs.pm/phoenix/overview.html
+* Docs: https://hexdocs.pm/phoenix
+* Forum: https://elixirforum.com/c/phoenix-forum
+* Source: https://github.com/phoenixframework/phoenix