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>

bootstrap/app.php

@@ -17,6 +17,7 @@ return Application::configure(basePath: dirname(__DIR__))
         ]);
         $middleware->alias([
             'admin' => \App\Http\Middleware\Admin::class,
+            'trainer' => \App\Http\Middleware\Trainer::class,
         ]);
     })
     ->withExceptions(function (Exceptions $exceptions): void {