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/database/migrations/2026_04_10_193837_add_cancellation_tracking_to_shift_assignments.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('shift_assignments', function (Blueprint $table) {
+            $table->ulid('cancelled_by')->nullable()->after('rejection_reason');
+            $table->string('cancellation_source')->nullable()->after('cancelled_by');
+            $table->timestamp('cancelled_at')->nullable()->after('cancellation_source');
+
+            $table->foreign('cancelled_by')->references('id')->on('users')->nullOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('shift_assignments', function (Blueprint $table) {
+            $table->dropForeign(['cancelled_by']);
+            $table->dropColumn(['cancelled_by', 'cancellation_source', 'cancelled_at']);
+        });
+    }
+};