refactor : simplify codebase and fix critical issues

Backend:
- Add MCP Serializer to centralize entity-to-array conversion (~300 lines deduped)
- Fix race condition in task/ticket number generation (SELECT FOR UPDATE + transaction)
- Add unique constraint on task (project_id, number) with migration
- Fix MIME type validation: use server-detected finfo instead of client-supplied type
- Add allowlist of permitted MIME types for uploads
- Fix TaskDocumentDownloadController: allow ROLE_CLIENT access, add priority:1
- Fix notification sent even when ticket status unchanged
- Remove redundant exception constructors
- Simplify services (BookStackApi double fetch, TokenEncryptor, GiteaApi)
- Consolidate duplicate checks in processors

Frontend:
- Fix useApi isHandlingUnauthorized scope (module-level to prevent double 401 redirect)
- Fix client-tickets toast key copy-paste bug
- Merge duplicated tasks service methods (getByProject + getByProjectArchived)
- Extract shared uploadWithRelation helper in task-documents service
- Extract formatFileSize utility from duplicated component code
- Extract status transition logic into useClientTicketHelpers composable
- Remove dead code (unused router, handleLogout, empty script blocks)
- Merge duplicate watchers and onMounted calls
- Normalize arrow functions to function declarations per convention

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

src/Repository/TaskRepository.php

@@ -9,6 +9,9 @@ use App\Entity\Task;
 use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
 use Doctrine\Persistence\ManagerRegistry;
 
+/**
+ * @extends ServiceEntityRepository<Task>
+ */
 class TaskRepository extends ServiceEntityRepository
 {
     public function __construct(ManagerRegistry $registry)
@@ -16,16 +19,19 @@ class TaskRepository extends ServiceEntityRepository
         parent::__construct($registry, Task::class);
     }
 
-    public function findMaxNumberByProject(Project $project): int
+    /**
+     * Returns the max task number for a project, using a row-level lock
+     * to prevent race conditions when creating tasks concurrently.
+     */
+    public function findMaxNumberByProjectForUpdate(Project $project): int
     {
-        $result = $this->createQueryBuilder('t')
-            ->select('MAX(t.number)')
-            ->where('t.project = :project')
-            ->setParameter('project', $project)
-            ->getQuery()
-            ->getSingleScalarResult()
-        ;
+        $conn = $this->getEntityManager()->getConnection();
 
-        return (int) ($result ?? 0);
+        $result = $conn->fetchOne(
+            'SELECT COALESCE(MAX(number), 0) FROM task WHERE project_id = :project FOR UPDATE',
+            ['project' => $project->getId()],
+        );
+
+        return (int) $result;
     }
 }