feat: smart re-assignment with cancellation source tracking

Add cancelled_by, cancellation_source (organiser|volunteer|system), and
cancelled_at columns to shift_assignments. Cancel flow now records who
cancelled and why. Assign flow reactivates existing cancelled/rejected
records instead of creating duplicates, preventing UNIQUE constraint
violations. Assignable-persons endpoint returns previous_assignment data
for contextual UI indicators. Frontend shows cancellation source labels,
previous assignment history in assign dialog, and "Opnieuw toewijzen"
buttons with volunteer-cancelled confirmation dialogs.

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

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

@@ -120,7 +120,7 @@ final class ShiftAssignmentController extends Controller
             ->get()
             ->keyBy('person_id');
 
-        // Get all assignments for THIS shift in one query
+        // Get active (non-cancelled/rejected) assignments for THIS shift
         $alreadyAssigned = ShiftAssignment::where('shift_id', $shiftId)
             ->whereNotIn('status', [
                 ShiftAssignmentStatus::REJECTED,
@@ -129,15 +129,25 @@ final class ShiftAssignmentController extends Controller
             ->pluck('person_id')
             ->flip();
 
+        // Get previous cancelled/rejected assignments on THIS shift
+        $previousAssignments = ShiftAssignment::where('shift_id', $shiftId)
+            ->whereIn('status', [
+                ShiftAssignmentStatus::CANCELLED,
+                ShiftAssignmentStatus::REJECTED,
+            ])
+            ->get()
+            ->keyBy('person_id');
+
         $persons = Person::where('event_id', $festivalEventId)
             ->where('status', PersonStatus::APPROVED)
             ->with('crowdType')
             ->orderBy('name')
             ->get()
-            ->map(function (Person $person) use ($conflicts, $alreadyAssigned, $shiftId) {
+            ->map(function (Person $person) use ($conflicts, $alreadyAssigned, $previousAssignments, $shiftId) {
                 $isAlreadyAssigned = $alreadyAssigned->has($person->id);
                 $conflict = $conflicts->get($person->id);
                 $hasConflict = $conflict && $conflict->shift_id !== $shiftId;
+                $previous = $previousAssignments->get($person->id);
 
                 return [
                     'id' => $person->id,
@@ -158,6 +168,12 @@ final class ShiftAssignmentController extends Controller
                         'time' => $conflict->shift->timeSlot->start_time
                             . '–' . $conflict->shift->timeSlot->end_time,
                     ] : null,
+                    'previous_assignment' => $previous ? [
+                        'status' => $previous->status->value,
+                        'cancellation_source' => $previous->cancellation_source?->value,
+                        'cancelled_at' => $previous->cancelled_at?->toIso8601String(),
+                        'rejection_reason' => $previous->rejection_reason,
+                    ] : null,
                 ];
             })
             ->sortBy([