Phase 1 first chunk: inventory schema + DO droplet sync

Models:
- cloud_projects: arcadia-cloud's mirror of DO Projects, indexed by
  (provider, provider_id); tenant_id + purpose classify each project.
- cloud_resources: single unified resource table; kind-specific bits in
  attrs JSONB; first_seen_at / last_seen_at / stale_strike_count drive
  three-strike deletion.
- cloud_resource_events: append-only audit (discovered, updated, deleted,
  drift_detected, tagged, restored).

ArcadiaCloud.Cloud context owns the single upsert chokepoint that:
- inserts new with `discovered` event
- updates existing only when meaningful fields change
- restores tombstoned rows seen again
- bumps last_seen_at and resets strike count
mark_stale/3 implements the three-strike rule.

ArcadiaCloud.DigitalOcean.Client is a Req wrapper with auto-pagination.
Per-purpose token resolution via .Tokens (phase 1: env DO_API_TOKEN;
phase 2: vault). Per project_arcadia_cloud memory the long-term shape
is one PAT per queue purpose for rate-limit isolation.

ArcadiaCloud.Sync.Bootstrap ensures the skyai-internal DO Project exists
on first sync, idempotent thereafter. ArcadiaCloud.Sync.DropletsWorker
runs full droplet sync on the cloud_sync_full Oban queue.

InventoryController wired to real data: platform_admin sees all,
tenants see only their scope.

Live smoke test against real DO: 5 droplets synced; skyai-internal
project auto-created; events written; endpoint returns scoped results.

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

lib/arcadia_cloud/cloud/cloud_resource_event.ex

@@ -0,0 +1,27 @@
+defmodule ArcadiaCloud.Cloud.CloudResourceEvent do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key {:id, :binary_id, autogenerate: true}
+  @foreign_key_type :binary_id
+
+  schema "cloud_resource_events" do
+    field :event, :string
+    field :before, :map
+    field :after, :map
+    field :source, :string
+    field :occurred_at, :utc_datetime
+
+    belongs_to :resource, ArcadiaCloud.Cloud.CloudResource
+  end
+
+  @required ~w(resource_id event source occurred_at)a
+  @optional ~w(before after)a
+
+  def changeset(event, attrs) do
+    event
+    |> cast(attrs, @required ++ @optional)
+    |> validate_required(@required)
+    |> validate_inclusion(:event, ~w(discovered updated deleted drift_detected tagged restored))
+  end
+end