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/Entity/Task.php

@@ -35,6 +35,8 @@ use Symfony\Component\Serializer\Attribute\Groups;
 #[ApiFilter(SearchFilter::class, properties: ['project' => 'exact', 'group' => 'exact', 'assignee' => 'exact', 'priority' => 'exact', 'effort' => 'exact', 'tags' => 'exact', 'status' => 'exact'])]
 #[ApiFilter(BooleanFilter::class, properties: ['archived'])]
 #[ORM\Entity(repositoryClass: TaskRepository::class)]
+#[ORM\Table(name: 'task')]
+#[ORM\UniqueConstraint(name: 'uniq_task_project_number', columns: ['project_id', 'number'])]
 class Task
 {
     #[ORM\Id]