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/Models/ShiftAssignment.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Models;
 
+use App\Enums\CancellationSource;
 use App\Enums\ShiftAssignmentStatus;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
@@ -29,6 +30,9 @@ final class ShiftAssignment extends Model
         'approved_by',
         'approved_at',
         'rejection_reason',
+        'cancelled_by',
+        'cancellation_source',
+        'cancelled_at',
         'hours_expected',
         'hours_completed',
         'checked_in_at',
@@ -39,9 +43,11 @@ final class ShiftAssignment extends Model
     {
         return [
             'status' => ShiftAssignmentStatus::class,
+            'cancellation_source' => CancellationSource::class,
             'auto_approved' => 'boolean',
             'assigned_at' => 'datetime',
             'approved_at' => 'datetime',
+            'cancelled_at' => 'datetime',
             'checked_in_at' => 'datetime',
             'checked_out_at' => 'datetime',
             'hours_expected' => 'decimal:2',
@@ -74,6 +80,11 @@ final class ShiftAssignment extends Model
         return $this->belongsTo(User::class, 'approved_by');
     }
 
+    public function cancelledByUser(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'cancelled_by');
+    }
+
     public function scopeActive(Builder $query): Builder
     {
         return $query->whereIn('status', [