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/Http/Controllers/Auth/RegisteredUserController.php

@@ -7,6 +7,7 @@ use App\Models\User;
 use App\Notifications\NewUserRegistered;
 use App\Rules\NotDisposableEmail;
 use App\Rules\ReservedIdentifier;
+use App\Support\SafeNotifier;
 use Illuminate\Auth\Events\Registered;
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\Hash;
@@ -49,7 +50,7 @@ class RegisteredUserController extends Controller
 
         // Notify all super admins of the new registration
         User::where('role', 'super_admin')->each(function (User $admin) use ($user) {
-            $admin->notify(new NewUserRegistered($user));
+            SafeNotifier::send($admin, new NewUserRegistered($user));
         });
 
         auth()->login($user);