fix(backend) : add validation constraints and fix concurrent numbering

- Add Assert\Choice on ClientTicket type and status with typed constants
- Add Assert\Url on GiteaConfiguration, BookStackConfiguration, TaskBookStackLink, ClientTicket
- Fix concurrent task/ticket numbering: use pg_advisory_xact_lock instead of FOR UPDATE with MAX()
- Wrap CreateTaskTool numbering in transaction
- Harmonize repository contracts: both return max number, caller adds +1

Tickets: T-004, T-008, T-011, T-012

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

src/Entity/BookStackConfiguration.php

@@ -6,6 +6,7 @@ namespace App\Entity;
 
 use App\Repository\BookStackConfigurationRepository;
 use Doctrine\ORM\Mapping as ORM;
+use Symfony\Component\Validator\Constraints as Assert;
 
 #[ORM\Entity(repositoryClass: BookStackConfigurationRepository::class)]
 class BookStackConfiguration
@@ -16,6 +17,7 @@ class BookStackConfiguration
     private ?int $id = null;
 
     #[ORM\Column(length: 255, nullable: true)]
+    #[Assert\Url]
     private ?string $url = null;
 
     #[ORM\Column(type: 'text', nullable: true)]