feat: shift assignment workflow with claim, approve, reject, cancel, and bulk approve

Implements the complete ShiftAssignment lifecycle:
- ShiftAssignmentStatus enum with allowed transitions
- ShiftAssignmentService with claim/assign/approve/reject/cancel/bulkApprove
- ShiftAssignmentController with event-scoped endpoints
- ShiftAssignmentPolicy (organizer + volunteer self-cancel)
- VolunteerAvailability model, controller, and sync endpoint
- Refactored ShiftController to delegate to service layer
- 31 workflow tests covering all paths and multi-tenancy

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

api/app/Models/ShiftAssignment.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace App\Models;
 
+use App\Enums\ShiftAssignmentStatus;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
@@ -36,11 +38,14 @@ final class ShiftAssignment extends Model
     protected function casts(): array
     {
         return [
+            'status' => ShiftAssignmentStatus::class,
             'auto_approved' => 'boolean',
             'assigned_at' => 'datetime',
             'approved_at' => 'datetime',
             'checked_in_at' => 'datetime',
             'checked_out_at' => 'datetime',
+            'hours_expected' => 'decimal:2',
+            'hours_completed' => 'decimal:2',
         ];
     }
 
@@ -58,4 +63,37 @@ final class ShiftAssignment extends Model
     {
         return $this->belongsTo(TimeSlot::class);
     }
+
+    public function assignedByUser(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'assigned_by');
+    }
+
+    public function approvedByUser(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'approved_by');
+    }
+
+    public function scopeActive(Builder $query): Builder
+    {
+        return $query->whereIn('status', [
+            ShiftAssignmentStatus::PENDING_APPROVAL,
+            ShiftAssignmentStatus::APPROVED,
+        ]);
+    }
+
+    public function scopeForStatus(Builder $query, ShiftAssignmentStatus $status): Builder
+    {
+        return $query->where('status', $status);
+    }
+
+    public function isCancellable(): bool
+    {
+        return $this->status->canTransitionTo(ShiftAssignmentStatus::CANCELLED);
+    }
+
+    public function isApprovable(): bool
+    {
+        return $this->status->canTransitionTo(ShiftAssignmentStatus::APPROVED);
+    }
 }