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>

lib/arcadia_cloud_web/telemetry.ex

@@ -0,0 +1,93 @@
+defmodule ArcadiaCloudWeb.Telemetry do
+  use Supervisor
+  import Telemetry.Metrics
+
+  def start_link(arg) do
+    Supervisor.start_link(__MODULE__, arg, name: __MODULE__)
+  end
+
+  @impl true
+  def init(_arg) do
+    children = [
+      # Telemetry poller will execute the given period measurements
+      # every 10_000ms. Learn more here: https://hexdocs.pm/telemetry_metrics
+      {:telemetry_poller, measurements: periodic_measurements(), period: 10_000}
+      # Add reporters as children of your supervision tree.
+      # {Telemetry.Metrics.ConsoleReporter, metrics: metrics()}
+    ]
+
+    Supervisor.init(children, strategy: :one_for_one)
+  end
+
+  def metrics do
+    [
+      # Phoenix Metrics
+      summary("phoenix.endpoint.start.system_time",
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.endpoint.stop.duration",
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.router_dispatch.start.system_time",
+        tags: [:route],
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.router_dispatch.exception.duration",
+        tags: [:route],
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.router_dispatch.stop.duration",
+        tags: [:route],
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.socket_connected.duration",
+        unit: {:native, :millisecond}
+      ),
+      sum("phoenix.socket_drain.count"),
+      summary("phoenix.channel_joined.duration",
+        unit: {:native, :millisecond}
+      ),
+      summary("phoenix.channel_handled_in.duration",
+        tags: [:event],
+        unit: {:native, :millisecond}
+      ),
+
+      # Database Metrics
+      summary("arcadia_cloud.repo.query.total_time",
+        unit: {:native, :millisecond},
+        description: "The sum of the other measurements"
+      ),
+      summary("arcadia_cloud.repo.query.decode_time",
+        unit: {:native, :millisecond},
+        description: "The time spent decoding the data received from the database"
+      ),
+      summary("arcadia_cloud.repo.query.query_time",
+        unit: {:native, :millisecond},
+        description: "The time spent executing the query"
+      ),
+      summary("arcadia_cloud.repo.query.queue_time",
+        unit: {:native, :millisecond},
+        description: "The time spent waiting for a database connection"
+      ),
+      summary("arcadia_cloud.repo.query.idle_time",
+        unit: {:native, :millisecond},
+        description:
+          "The time the connection spent waiting before being checked out for the query"
+      ),
+
+      # VM Metrics
+      summary("vm.memory.total", unit: {:byte, :kilobyte}),
+      summary("vm.total_run_queue_lengths.total"),
+      summary("vm.total_run_queue_lengths.cpu"),
+      summary("vm.total_run_queue_lengths.io")
+    ]
+  end
+
+  defp periodic_measurements do
+    [
+      # A module, function and arguments to be invoked periodically.
+      # This function must call :telemetry.execute/3 and a metric must be added above.
+      # {ArcadiaCloudWeb, :count_users, []}
+    ]
+  end
+end