Add multi-week workout programs

A Workout was a single flat session with no way to schedule several
across weeks/days as a structured plan (e.g. Push/Pull/Legs over 4
weeks) - every user started from a blank builder each time, and there
was no template concept at all.

Adds a Program (title, description, week count) that Workouts can
optionally belong to via nullable program_id/week_number/day_number
columns, reusing all existing workout and session-tracking
infrastructure rather than building a parallel system. The program show
page lays out the schedule week by week and derives progress (completed
vs total) from the user's existing WorkoutSession completions - no new
enrollment/progress-tracking concept needed. Deleting a program
unschedules its workouts (nullOnDelete) rather than destroying them.

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

app/Models/Program.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class Program extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'user_id',
+        'title',
+        'description',
+        'weeks',
+    ];
+
+    protected $casts = [
+        'weeks' => 'integer',
+    ];
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    public function workouts(): HasMany
+    {
+        return $this->hasMany(Workout::class)->orderBy('week_number')->orderBy('day_number');
+    }
+}