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.

config/leadmail.php

@@ -52,4 +52,55 @@ return [
     |
     */
     'auto_tenant' => env('LEADMAIL_AUTO_TENANT', true),
+
+    /*
+    |--------------------------------------------------------------------------
+    | Retries
+    |--------------------------------------------------------------------------
+    |
+    | How many times to retry a request that fails transiently (connection
+    | errors and 429/5xx responses) before giving up. The delay between
+    | attempts grows exponentially from the base delay (in milliseconds).
+    |
+    */
+    'retries' => env('LEADMAIL_RETRIES', 2),
+
+    'retry_delay' => env('LEADMAIL_RETRY_DELAY_MS', 200),
+
+    /*
+    |--------------------------------------------------------------------------
+    | Webhook Secret
+    |--------------------------------------------------------------------------
+    |
+    | The signing secret for verifying incoming failure webhooks. This must
+    | match the webhook secret shown for this client app in the leadMail admin
+    | dashboard. Set automatically by `php artisan leadmail:install`.
+    |
+    */
+    'webhook_secret' => env('LEADMAIL_WEBHOOK_SECRET'),
+
+    /*
+    |--------------------------------------------------------------------------
+    | Webhook Route
+    |--------------------------------------------------------------------------
+    |
+    | The path the SDK automatically registers to receive failure webhooks.
+    | The registered endpoint verifies the signature, logs failures, and fires
+    | a LeadMailWebhookReceived event you can listen for. Set to null to
+    | disable auto-registration and handle the route yourself.
+    |
+    */
+    'webhook_route' => env('LEADMAIL_WEBHOOK_ROUTE', '/webhooks/leadmail'),
+
+    /*
+    |--------------------------------------------------------------------------
+    | Webhook Route Middleware
+    |--------------------------------------------------------------------------
+    |
+    | Middleware applied to the auto-registered webhook route. The endpoint is
+    | already authenticated by HMAC signature, so it deliberately runs outside
+    | the "web" group (no CSRF, no session).
+    |
+    */
+    'webhook_middleware' => ['throttle:60,1'],
 ];