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/Services/ShiftAssignmentService.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Services;
 
+use App\Enums\CancellationSource;
 use App\Enums\ShiftAssignmentStatus;
 use App\Models\Person;
 use App\Models\Shift;
@@ -72,6 +73,44 @@ final class ShiftAssignmentService
         return DB::transaction(function () use ($shift, $person, $assignedBy): ShiftAssignment {
             $this->validateNoConflict($shift, $person);
 
+            // Check for existing cancelled/rejected assignment on THIS shift
+            $existing = ShiftAssignment::where('shift_id', $shift->id)
+                ->where('person_id', $person->id)
+                ->whereIn('status', [
+                    ShiftAssignmentStatus::CANCELLED,
+                    ShiftAssignmentStatus::REJECTED,
+                ])
+                ->first();
+
+            if ($existing) {
+                $previousStatus = $existing->status->value;
+
+                $existing->update([
+                    'status' => ShiftAssignmentStatus::APPROVED,
+                    'assigned_by' => $assignedBy->id,
+                    'assigned_at' => now(),
+                    'approved_by' => $assignedBy->id,
+                    'approved_at' => now(),
+                    'rejection_reason' => null,
+                    'cancelled_by' => null,
+                    'cancellation_source' => null,
+                    'cancelled_at' => null,
+                ]);
+
+                activity('shift_assignment')
+                    ->causedBy($assignedBy)
+                    ->performedOn($existing)
+                    ->withProperties([
+                        'previous_status' => $previousStatus,
+                        'person_name' => $person->name,
+                    ])
+                    ->log('shift_assignment.reactivated');
+
+                $this->updateShiftStatusIfFull($shift);
+
+                return $existing->fresh();
+            }
+
             // Log overbooking for audit trail (organizers may intentionally overbook)
             $filledSlots = $shift->shiftAssignments()
                 ->where('status', ShiftAssignmentStatus::APPROVED)
@@ -189,9 +228,12 @@ final class ShiftAssignmentService
     /**
      * @throws ValidationException
      */
-    public function cancel(ShiftAssignment $assignment, User $cancelledBy): ShiftAssignment
-    {
-        return DB::transaction(function () use ($assignment, $cancelledBy): ShiftAssignment {
+    public function cancel(
+        ShiftAssignment $assignment,
+        User $cancelledBy,
+        CancellationSource $source = CancellationSource::ORGANISER,
+    ): ShiftAssignment {
+        return DB::transaction(function () use ($assignment, $cancelledBy, $source): ShiftAssignment {
             $this->validateStatusTransition($assignment, ShiftAssignmentStatus::CANCELLED);
 
             $wasApproved = $assignment->status === ShiftAssignmentStatus::APPROVED;
@@ -199,6 +241,9 @@ final class ShiftAssignmentService
 
             $assignment->update([
                 'status' => ShiftAssignmentStatus::CANCELLED,
+                'cancelled_by' => $cancelledBy->id,
+                'cancellation_source' => $source,
+                'cancelled_at' => now(),
             ]);
 
             if ($wasApproved) {
@@ -211,6 +256,7 @@ final class ShiftAssignmentService
                 ->withProperties([
                     'old_status' => $oldStatus->value,
                     'new_status' => ShiftAssignmentStatus::CANCELLED->value,
+                    'source' => $source->value,
                 ])
                 ->log('shift_assignment.cancelled');