Build out the trainer role

is_trainer was captured at signup and stored, but nothing in the app
used it - no client roster, no way to assign anything, a dead flag that
misled users who checked it during registration.

Adds a trainer/client link (a trainer adds a client by the email they
registered with) and lets a trainer create workouts directly on a
client's account (assigned_by_trainer_id on workouts). Deliberately
scoped for privacy: a trainer only ever sees/manages the workouts they
authored and assigned - not a client's other private data (their own
workouts, weight log, health notes, session/completion history). That
boundary needs a consent or notification system to relax safely, which
doesn't exist yet, so it's left for a follow-up rather than building it
half-safe.

WorkoutController's authorization now distinguishes "manage" (owner or
assigning trainer - view, add/edit exercises) from "own" (owner only -
delete), so a trainer can build out an assigned workout's exercises
after creating it, but can't delete a client's workout out from under
them.

Found a real bug via live testing before committing: the client roster
page eager-loaded `assignedWorkouts` (keyed by assigned_by_trainer_id)
on each client record, which is the trainer's own perspective, not the
client's - so it silently showed "no workouts assigned" even after
assigning one. Fixed to eager-load `workouts` filtered by
assigned_by_trainer_id instead, with a regression test.

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

app/Models/Workout.php

@@ -20,6 +20,7 @@ class Workout extends Model
         'program_id',
         'week_number',
         'day_number',
+        'assigned_by_trainer_id',
     ];
 
     protected $casts = [
@@ -38,6 +39,11 @@ class Workout extends Model
         return $this->belongsTo(Program::class);
     }
 
+    public function assignedByTrainer(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'assigned_by_trainer_id');
+    }
+
     public function items(): HasMany
     {
         return $this->hasMany(WorkoutExercise::class)->orderBy('position');