feat(mail): make notification failures non-fatal

Add App\Support\SafeNotifier that wraps $notifiable->notify() in a
Throwable guard with a logged warning, so SMTP outages (the current
Gmail 535 errors) no longer propagate as a 500 to the user. Apply to
the two hot paths that were still unprotected: CommentController
(new comment, reply, like) and RegisteredUserController (super-admin
registration alert). User::sendEmailVerificationNotification() is
wrapped inline for the same reason.

Also add docs/mail-setup.md covering the three delivery options:
Gmail app password (quick), Mailgun, Postmark — with the exact .env
keys for each.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/Support/SafeNotifier.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Support;
+
+use Illuminate\Notifications\Notification;
+use Illuminate\Support\Facades\Log;
+
+/**
+ * Notification failures (SMTP down, Gmail 535, Mailgun 4xx, etc.) must never
+ * crash a user-facing request. Everywhere we previously did
+ *
+ *     $user->notify(new SomeNotification(...));
+ *
+ * we now do
+ *
+ *     SafeNotifier::send($user, new SomeNotification(...));
+ *
+ * which runs the same code, catches any Throwable, and writes a warning line
+ * so ops still has the signal.
+ */
+class SafeNotifier
+{
+    /**
+     * @param mixed $notifiable  Anything with ->notify(), usually a User.
+     */
+    public static function send(mixed $notifiable, Notification $notification): void
+    {
+        if (! $notifiable || ! is_object($notifiable) || ! method_exists($notifiable, 'notify')) {
+            return;
+        }
+
+        try {
+            $notifiable->notify($notification);
+        } catch (\Throwable $e) {
+            Log::warning('notification send failed', [
+                'notification' => $notification::class,
+                'notifiable'   => $notifiable::class,
+                'notifiable_id'=> $notifiable->getKey() ?? null,
+                'error'        => $e->getMessage(),
+            ]);
+        }
+    }
+}