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.

src/Http/Controllers/LeadMailWebhookController.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace LeadM\LeadMail\Http\Controllers;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Response;
+use Illuminate\Support\Facades\Log;
+use LeadM\LeadMail\Events\LeadMailWebhookReceived;
+use LeadM\LeadMail\Exceptions\InvalidWebhookSignatureException;
+use LeadM\LeadMail\Webhooks\LeadMailWebhook;
+
+/**
+ * Default receiver for LeadMail webhooks, auto-registered by the service
+ * provider. Verifies the signature, logs failures, and dispatches a
+ * LeadMailWebhookReceived event for custom handling.
+ */
+class LeadMailWebhookController
+{
+    public function __invoke(Request $request, LeadMailWebhook $webhook): Response
+    {
+        try {
+            $event = $webhook->parse($request);
+        } catch (InvalidWebhookSignatureException) {
+            Log::warning('LeadMail webhook rejected: invalid or missing signature.');
+
+            return response('Invalid signature.', 403);
+        }
+
+        if ($event->isFailure()) {
+            Log::warning('LeadMail reported a failed email.', [
+                'log_id' => $event->logId,
+                'to' => $event->to,
+                'error_code' => $event->errorCode,
+                'error_message' => $event->errorMessage,
+            ]);
+        }
+
+        event(new LeadMailWebhookReceived($event));
+
+        return response()->noContent();
+    }
+}