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/Http/Controllers/TrainerController.php

@@ -0,0 +1,92 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Models\User;
+use App\Models\Workout;
+use Illuminate\Http\RedirectResponse;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Auth;
+use Illuminate\View\View;
+
+class TrainerController extends Controller
+{
+    public function clients(): View
+    {
+        $trainer = Auth::user();
+        // Note: `workouts` (user_id) is what a client was assigned, not `assignedWorkouts`
+        // (assigned_by_trainer_id), which belongs to the trainer's own perspective.
+        $clients = $trainer->clients()
+            ->with(['workouts' => function ($q) use ($trainer) {
+                $q->where('assigned_by_trainer_id', $trainer->id)->latest();
+            }])
+            ->orderBy('name')
+            ->get();
+
+        return view('trainer.clients', compact('clients'));
+    }
+
+    public function addClient(Request $request): RedirectResponse
+    {
+        $trainer = Auth::user();
+        $data = $request->validate([
+            'email' => ['required', 'email', 'exists:users,email'],
+        ]);
+
+        $client = User::where('email', $data['email'])->firstOrFail();
+
+        if ($client->id === $trainer->id) {
+            return back()->withErrors(['email' => 'You cannot add yourself as a client.']);
+        }
+
+        if ($trainer->clients()->where('client_id', $client->id)->exists()) {
+            return back()->withErrors(['email' => 'This person is already one of your clients.']);
+        }
+
+        $trainer->clients()->attach($client->id);
+
+        return back()->with('status', 'Client added');
+    }
+
+    public function removeClient(User $client): RedirectResponse
+    {
+        Auth::user()->clients()->detach($client->id);
+
+        return back()->with('status', 'Client removed');
+    }
+
+    public function createWorkout(User $client): View
+    {
+        $this->authorizeClient($client);
+
+        return view('trainer.assign-workout', compact('client'));
+    }
+
+    public function storeWorkout(Request $request, User $client): RedirectResponse
+    {
+        $this->authorizeClient($client);
+
+        $data = $request->validate([
+            'title' => ['required', 'string', 'max:255'],
+            'notes' => ['nullable', 'string'],
+            'estimated_minutes' => ['nullable', 'integer', 'min:1', 'max:600'],
+        ]);
+
+        $workout = Workout::create([
+            'user_id' => $client->id,
+            'assigned_by_trainer_id' => Auth::id(),
+            'title' => $data['title'],
+            'notes' => $data['notes'] ?? null,
+            'estimated_minutes' => $data['estimated_minutes'] ?? null,
+        ]);
+
+        return redirect()->route('trainer.clients')->with('status', "Workout assigned to {$client->name}");
+    }
+
+    protected function authorizeClient(User $client): void
+    {
+        if (!Auth::user()->clients()->where('client_id', $client->id)->exists()) {
+            abort(403);
+        }
+    }
+}