Add inactivity reminder emails

No app/Notifications existed beyond Laravel's stock email verification
and password reset - nothing brought lapsed users back. Adds a queued
InactivityReminder mail notification and a scheduled command
(app:send-inactivity-reminders, daily at 09:00) that emails verified
users with no completed workout session in the last 5 days, at most
once per 7-day cooldown so it can't spam. Skips brand-new signups so it
doesn't nag someone who registered an hour ago.

Verified the full pipeline live: scheduled command queues the
notification (QUEUE_CONNECTION=database), a queue worker processes it,
and the rendered email lands in the log driver with a working link back
to /workouts.

Production still needs cron actually invoking `php artisan schedule:run`
every minute and a running queue worker - that's part of the pending
deployment setup, not something this change can provide on its own.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Notifications/InactivityReminder.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Notifications;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Notifications\Messages\MailMessage;
+use Illuminate\Notifications\Notification;
+
+class InactivityReminder extends Notification implements ShouldQueue
+{
+    use Queueable;
+
+    /**
+     * Get the notification's delivery channels.
+     *
+     * @return array<int, string>
+     */
+    public function via(object $notifiable): array
+    {
+        return ['mail'];
+    }
+
+    /**
+     * Get the mail representation of the notification.
+     */
+    public function toMail(object $notifiable): MailMessage
+    {
+        return (new MailMessage)
+            ->subject("We haven't seen you in a while, {$notifiable->name}")
+            ->line('It has been a few days since your last completed workout.')
+            ->line('A short session today is better than a perfect one you keep putting off.')
+            ->action('Start a Workout', route('workouts.index'))
+            ->line('See you back soon!');
+    }
+}