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/Shift/ShiftTest.php

@@ -284,7 +284,7 @@ class ShiftTest extends TestCase
         $response->assertCreated();
     }
 
-    public function test_assign_full_shift_returns_422(): void
+    public function test_assign_full_shift_allows_overbooking(): void
     {
         $shift = Shift::factory()->create([
             'festival_section_id' => $this->section->id,
@@ -317,7 +317,13 @@ class ShiftTest extends TestCase
             'person_id' => $person2->id,
         ]);
 
-        $response->assertUnprocessable();
+        $response->assertCreated();
+
+        $this->assertDatabaseHas('shift_assignments', [
+            'shift_id' => $shift->id,
+            'person_id' => $person2->id,
+            'status' => 'approved',
+        ]);
     }
 
     public function test_claim_no_claimable_slots_returns_422(): void