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/Exception/BookStackApiException.php

@@ -5,12 +5,5 @@ declare(strict_types=1);
 namespace App\Exception;
 
 use RuntimeException;
-use Throwable;
 
-final class BookStackApiException extends RuntimeException
-{
-    public function __construct(string $message, int $code = 0, ?Throwable $previous = null)
-    {
-        parent::__construct($message, $code, $previous);
-    }
-}
+final class BookStackApiException extends RuntimeException {}

src/Exception/GiteaApiException.php

@@ -5,12 +5,5 @@ declare(strict_types=1);
 namespace App\Exception;
 
 use RuntimeException;
-use Throwable;
 
-final class GiteaApiException extends RuntimeException
-{
-    public function __construct(string $message, int $code = 0, ?Throwable $previous = null)
-    {
-        parent::__construct($message, $code, $previous);
-    }
-}
+final class GiteaApiException extends RuntimeException {}