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>

database/migrations/2026_08_10_093017_add_last_reminder_sent_at_to_users_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            $table->timestamp('last_reminder_sent_at')->nullable()->after('minutes_per_session');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            $table->dropColumn('last_reminder_sent_at');
+        });
+    }
+};