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/Unit/ServerPlaceholderIpTest.php

@@ -0,0 +1,67 @@
+<?php
+
+use App\Models\PrivateKey;
+use App\Models\Server;
+use App\Models\Team;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+uses(TestCase::class, RefreshDatabase::class);
+
+function createServerForPlaceholderIpTest(string $ip): Server
+{
+    $team = Team::create([
+        'name' => 'Test Team',
+        'personal_team' => false,
+    ]);
+    $privateKey = PrivateKey::factory()->create(['team_id' => $team->id]);
+
+    return Server::factory()->create([
+        'team_id' => $team->id,
+        'private_key_id' => $privateKey->id,
+        'ip' => $ip,
+    ]);
+}
+
+it('detects placeholder IPs', function () {
+    $server = createServerForPlaceholderIpTest(Server::PLACEHOLDER_IP);
+
+    foreach ([Server::PLACEHOLDER_IP, '0.0.0.0', '::'] as $ip) {
+        $server->update(['ip' => $ip]);
+
+        expect($server->fresh()->hasPlaceholderIp())->toBeTrue();
+    }
+});
+
+it('does not treat a real IP as a placeholder', function () {
+    $server = createServerForPlaceholderIpTest('203.0.113.5');
+
+    expect($server->hasPlaceholderIp())->toBeFalse();
+});
+
+it('backfills a placeholder IP with the real address', function () {
+    $server = createServerForPlaceholderIpTest(Server::PLACEHOLDER_IP);
+
+    expect($server->backfillPlaceholderIp('203.0.113.5'))->toBeTrue();
+    expect($server->fresh()->ip)->toBe('203.0.113.5');
+});
+
+it('does not overwrite a real IP when backfilling', function () {
+    $server = createServerForPlaceholderIpTest('203.0.113.5');
+
+    expect($server->backfillPlaceholderIp('198.51.100.9'))->toBeFalse();
+    expect($server->fresh()->ip)->toBe('203.0.113.5');
+});
+
+it('ignores a missing IP when backfilling', function () {
+    $server = createServerForPlaceholderIpTest(Server::PLACEHOLDER_IP);
+
+    expect($server->backfillPlaceholderIp(null))->toBeFalse();
+    expect($server->fresh()->ip)->toBe(Server::PLACEHOLDER_IP);
+});
+
+it('skips servers with a placeholder IP in scheduled jobs', function () {
+    $server = createServerForPlaceholderIpTest(Server::PLACEHOLDER_IP);
+
+    expect($server->skipServer())->toBeTrue();
+});