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/tests/Feature/Shift/ShiftTest.php

@@ -207,6 +207,7 @@ class ShiftTest extends TestCase
             'time_slot_id' => $this->timeSlot->id,
             'slots_total' => 4,
             'allow_overlap' => false,
+            'status' => 'open',
         ]);
 
         $person = Person::factory()->create([
@@ -229,6 +230,7 @@ class ShiftTest extends TestCase
             'time_slot_id' => $this->timeSlot->id,
             'slots_total' => 4,
             'allow_overlap' => false,
+            'status' => 'open',
         ]);
 
         Sanctum::actingAs($this->orgAdmin);
@@ -247,6 +249,7 @@ class ShiftTest extends TestCase
             'time_slot_id' => $this->timeSlot->id,
             'slots_total' => 4,
             'allow_overlap' => true,
+            'status' => 'open',
         ]);
 
         $person = Person::factory()->create([
@@ -269,6 +272,7 @@ class ShiftTest extends TestCase
             'time_slot_id' => $this->timeSlot->id,
             'slots_total' => 4,
             'allow_overlap' => true,
+            'status' => 'open',
         ]);
 
         Sanctum::actingAs($this->orgAdmin);
@@ -286,6 +290,7 @@ class ShiftTest extends TestCase
             'festival_section_id' => $this->section->id,
             'time_slot_id' => $this->timeSlot->id,
             'slots_total' => 1,
+            'status' => 'open',
         ]);
 
         $person1 = Person::factory()->create([