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/TaskBookStackLink.php

@@ -7,6 +7,7 @@ namespace App\Entity;
 use App\Repository\TaskBookStackLinkRepository;
 use DateTimeImmutable;
 use Doctrine\ORM\Mapping as ORM;
+use Symfony\Component\Validator\Constraints as Assert;
 
 #[ORM\Entity(repositoryClass: TaskBookStackLinkRepository::class)]
 #[ORM\UniqueConstraint(name: 'UNIQ_task_bookstack_link', columns: ['task_id', 'bookstack_id', 'bookstack_type'])]
@@ -31,6 +32,7 @@ class TaskBookStackLink
     private string $title;
 
     #[ORM\Column(length: 500)]
+    #[Assert\Url]
     private string $url;
 
     #[ORM\Column]