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/Models/User.php

@@ -74,6 +74,11 @@ class User extends Authenticatable
         return $this->hasMany(Program::class);
     }
 
+    public function workoutSessions()
+    {
+        return $this->hasMany(WorkoutSession::class);
+    }
+
     // Trainer/client relationships
     public function clients()
     {