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/Enums/CancellationSource.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums;
+
+enum CancellationSource: string
+{
+    case ORGANISER = 'organiser';
+    case VOLUNTEER = 'volunteer';
+    case SYSTEM = 'system';
+}