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/User.php

@@ -69,6 +69,11 @@ class User extends Authenticatable
         return $this->hasMany(Workout::class);
     }
 
+    public function programs()
+    {
+        return $this->hasMany(Program::class);
+    }
+
     public function favoriteExercises()
     {
         return $this->belongsToMany(Exercise::class, 'exercise_favorites')->withTimestamps();