feat(api): enrich TimeSlotResource with shift statistics

Add shifts_count, total_slots, filled_slots, and sections_count computed
fields to TimeSlotResource. Update TimeSlotController to eager-load shifts
with assignment counts for aggregate calculations. Includes test verifying
only approved assignments count towards filled_slots.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Http/Controllers/Api/V1/TimeSlotController.php

@@ -20,11 +20,24 @@ final class TimeSlotController extends Controller
     {
         Gate::authorize('viewAny', [TimeSlot::class, $event]);
 
-        $timeSlots = $event->timeSlots()->orderBy('date')->orderBy('start_time')->get();
+        $timeSlots = $event->timeSlots()
+            ->withCount('shifts')
+            ->with(['shifts' => fn ($q) => $q->withCount([
+                'shiftAssignments as assignments_count' => fn ($q) => $q->where('status', 'approved'),
+            ])])
+            ->orderBy('date')
+            ->orderBy('start_time')
+            ->get();
 
         if ($event->isSubEvent() && request()->boolean('include_parent') && $event->parent_event_id) {
             $parentTimeSlots = TimeSlot::where('event_id', $event->parent_event_id)
-                ->with('event')
+                ->withCount('shifts')
+                ->with([
+                    'event',
+                    'shifts' => fn ($q) => $q->withCount([
+                        'shiftAssignments as assignments_count' => fn ($q) => $q->where('status', 'approved'),
+                    ]),
+                ])
                 ->orderBy('date')
                 ->orderBy('start_time')
                 ->get();