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>

resources/views/workouts/index.blade.php

@@ -23,6 +23,9 @@
                     <div class="rounded-2xl ring-1 ring-gray-200 dark:ring-gray-700 bg-white dark:bg-gray-800 p-5 flex flex-col">
                         <div class="flex-1">
                             <h3 class="text-lg font-semibold text-gray-900 dark:text-white">{{ $workout->title }}</h3>
+                            @if($workout->assignedByTrainer)
+                                <span class="mt-1 inline-flex items-center rounded-full bg-indigo-100 dark:bg-indigo-900/40 text-indigo-700 dark:text-indigo-300 px-2.5 py-0.5 text-xs font-medium">Assigned by {{ $workout->assignedByTrainer->name }}</span>
+                            @endif
                             @if($workout->estimated_minutes)
                                 <p class="mt-1 text-sm text-gray-600 dark:text-gray-300">~ {{ $workout->estimated_minutes }} min</p>
                             @endif

resources/views/workouts/show.blade.php

@@ -23,6 +23,9 @@
                             </form>
                         @endauth
                     </div>
+                    @if($workout->assignedByTrainer)
+                        <span class="mt-2 inline-flex items-center rounded-full bg-indigo-100 dark:bg-indigo-900/40 text-indigo-700 dark:text-indigo-300 px-2.5 py-0.5 text-xs font-medium">Assigned by {{ $workout->assignedByTrainer->name }}</span>
+                    @endif
                     <p class="mt-1 text-gray-600 dark:text-gray-300">@if($workout->estimated_minutes) ~ {{ $workout->estimated_minutes }} min · @endif {{ $workout->items->count() }} exercises</p>
                 </div>
                 <div class="flex items-center gap-3">