fix(servers): retain cloud instances awaiting IPs

Persist DigitalOcean, Hetzner, and Vultr servers before public IP
assignment, then backfill placeholder addresses from provider state.

Treat partial Sentinel snapshots as non-authoritative and document the
destinations API with OpenAPI schemas.

tests/Feature/MoveResourceApiTest.php

@@ -20,7 +20,10 @@ use Illuminate\Support\Facades\Log;
 uses(RefreshDatabase::class);
 
 beforeEach(function () {
-    InstanceSettings::create(['id' => 0, 'is_api_enabled' => true]);
+    InstanceSettings::unguarded(fn () => InstanceSettings::updateOrCreate(
+        ['id' => 0],
+        ['is_api_enabled' => true],
+    ));
 
     $this->team = Team::factory()->create();
     $this->user = User::factory()->create();