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/State/ClientTicketNumberProcessor.php

@@ -53,7 +53,8 @@ final readonly class ClientTicketNumberProcessor implements ProcessorInterface
 
         $now = new DateTimeImmutable();
 
-        $data->setNumber($this->clientTicketRepository->findNextNumberForProjectForUpdate($project));
+        $maxNumber = $this->clientTicketRepository->findMaxNumberByProjectForUpdate($project);
+        $data->setNumber($maxNumber + 1);
         $data->setSubmittedBy($user);
         $data->setStatus('new');
         $data->setCreatedAt($now);