feat: allow organizer overbooking with confirmation dialog

Remove capacity and status validation from organizer assign flow so
organizers can intentionally overbook shifts. Log overbooked assignments
for audit trail. Volunteer claims still enforce hard limits. Frontend
shows a warning banner when a shift is full and requires confirmation
before overbooking.

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

api/tests/Feature/Api/V1/ShiftAssignmentWorkflowTest.php

@@ -289,7 +289,7 @@ class ShiftAssignmentWorkflowTest extends TestCase
         $response->assertCreated();
     }
 
-    public function test_assign_rejected_when_capacity_full(): void
+    public function test_assign_allows_overbooking_when_capacity_full(): void
     {
         $shift = $this->createOpenShift(['slots_total' => 1]);
 
@@ -310,7 +310,13 @@ class ShiftAssignmentWorkflowTest extends TestCase
             ['person_id' => $this->person->id],
         );
 
-        $response->assertUnprocessable();
+        $response->assertCreated();
+
+        $this->assertDatabaseHas('shift_assignments', [
+            'shift_id' => $shift->id,
+            'person_id' => $this->person->id,
+            'status' => 'approved',
+        ]);
     }
 
     public function test_assign_rejected_with_conflict(): void