Add typed error handling, retries, and webhook receiver

- Typed exceptions: LeadMailException base with LeadMailRequestException
  (structured statusCode/errorCode/logId/validationErrors) and
  LeadMailConnectionException; sendEmail/getDomains now throw these instead
  of raw Guzzle exceptions, and a malformed body is no longer a silent null.
- Automatic retry with exponential backoff on idempotent calls
  (getDomains, verifyEmail); sends are never retried to avoid duplicates.
- Webhook receiver: auto-registered route + LeadMailWebhookController that
  verifies the HMAC signature, logs failures, and dispatches a
  LeadMailWebhookReceived event. WebhookSignature/WebhookEvent/LeadMailWebhook
  helpers for manual handling.
- Webhook self-registration client methods (registerWebhook/getWebhook/
  deleteWebhook) and a promptless `leadmail:install` command that registers
  the URL and writes LEADMAIL_WEBHOOK_SECRET to .env.
- Null-safe client binding when LEADMAIL_TOKEN is unset.
- Test suite (Pest + Testbench) covering all of the above.

tests/Feature/InstallCommandTest.php

@@ -0,0 +1,118 @@
+<?php
+
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use LeadM\LeadMail\LeadMailClient;
+
+/**
+ * Bind a client backed by canned HTTP responses into the container so the
+ * command resolves it instead of making real requests.
+ *
+ * @param  array<int, mixed>  $queue
+ */
+function bindMockedClient(array $queue): void
+{
+    app()->instance(LeadMailClient::class, new LeadMailClient(
+        baseUrl: 'https://mail.test',
+        token: 'lm_test',
+        retryDelayMs: 0,
+        handlerStack: HandlerStack::create(new MockHandler($queue)),
+    ));
+}
+
+/**
+ * Point the application at a throwaway .env file and return its path.
+ */
+function useTempEnv(string $contents = "APP_NAME=Test\n"): string
+{
+    $dir = sys_get_temp_dir().'/leadmail-test-'.uniqid();
+    mkdir($dir);
+    file_put_contents($dir.'/.env', $contents);
+    app()->useEnvironmentPath($dir);
+
+    return $dir.'/.env';
+}
+
+function registrationResponse(?string $secret): Response
+{
+    $data = ['url' => 'https://app.test/webhooks/leadmail', 'has_secret' => true];
+
+    if ($secret !== null) {
+        $data['secret'] = $secret;
+    }
+
+    return new Response(200, [], json_encode(['success' => true, 'data' => $data]));
+}
+
+it('derives the webhook url from app.url and the configured route without prompting', function () {
+    config()->set('leadmail.token', 'lm_test');
+    config()->set('app.url', 'https://helios.test');
+    config()->set('leadmail.webhook_route', '/webhooks/leadmail');
+    useTempEnv();
+
+    $mock = new MockHandler([registrationResponse('whsec_x')]);
+    app()->instance(LeadMailClient::class, new LeadMailClient(
+        baseUrl: 'https://mail.test',
+        token: 'lm_test',
+        retryDelayMs: 0,
+        handlerStack: HandlerStack::create($mock),
+    ));
+
+    $this->artisan('leadmail:install', ['--no-publish' => true])->assertSuccessful();
+
+    $sent = json_decode((string) $mock->getLastRequest()->getBody(), true);
+    expect($sent['url'])->toBe('https://helios.test/webhooks/leadmail');
+});
+
+it('registers the webhook and writes the secret to .env', function () {
+    config()->set('leadmail.token', 'lm_test');
+    $envPath = useTempEnv();
+    bindMockedClient([registrationResponse('whsec_written')]);
+
+    $this->artisan('leadmail:install', [
+        '--url' => 'https://app.test/webhooks/leadmail',
+        '--no-publish' => true,
+    ])->assertSuccessful();
+
+    expect(file_get_contents($envPath))->toContain('LEADMAIL_WEBHOOK_SECRET=whsec_written');
+});
+
+it('replaces an existing secret line instead of duplicating it', function () {
+    config()->set('leadmail.token', 'lm_test');
+    $envPath = useTempEnv("APP_NAME=Test\nLEADMAIL_WEBHOOK_SECRET=whsec_old\n");
+    bindMockedClient([registrationResponse('whsec_new')]);
+
+    $this->artisan('leadmail:install', [
+        '--url' => 'https://app.test/webhooks/leadmail',
+        '--no-publish' => true,
+    ])->assertSuccessful();
+
+    $contents = file_get_contents($envPath);
+    expect($contents)->toContain('whsec_new')
+        ->and($contents)->not->toContain('whsec_old')
+        ->and(substr_count($contents, 'LEADMAIL_WEBHOOK_SECRET='))->toBe(1);
+});
+
+it('does not touch .env when the url is updated without a new secret', function () {
+    config()->set('leadmail.token', 'lm_test');
+    $envPath = useTempEnv();
+    bindMockedClient([registrationResponse(null)]);
+
+    $this->artisan('leadmail:install', [
+        '--url' => 'https://app.test/webhooks/leadmail',
+        '--no-publish' => true,
+    ])->assertSuccessful();
+
+    expect(file_get_contents($envPath))->not->toContain('LEADMAIL_WEBHOOK_SECRET');
+});
+
+it('fails when the api token is not configured', function () {
+    config()->set('leadmail.token', null);
+    useTempEnv();
+
+    $this->artisan('leadmail:install', [
+        '--url' => 'https://app.test/webhooks/leadmail',
+        '--no-publish' => true,
+    ])->assertFailed();
+});