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>

frontend/components/task/TaskDocumentList.vue

@@ -28,7 +28,7 @@
                 <!-- File info -->
                 <div class="min-w-0 flex-1">
                     <p class="truncate text-xs font-medium text-neutral-700">{{ doc.originalName }}</p>
-                    <p class="text-xs text-neutral-400">{{ formatSize(doc.size) }}</p>
+                    <p class="text-xs text-neutral-400">{{ formatFileSize(doc.size) }}</p>
                 </div>
 
                 <!-- Delete button -->
@@ -47,6 +47,7 @@
 <script setup lang="ts">
 import type { TaskDocument } from '~/services/dto/task-document'
 import { useTaskDocumentService } from '~/services/task-documents'
+import { formatFileSize } from '~/utils/format'
 
 defineProps<{
     documents: TaskDocument[]
@@ -72,9 +73,4 @@ function getIconForMime(mimeType: string): string {
     return 'heroicons:paper-clip'
 }
 
-function formatSize(bytes: number): string {
-    if (bytes < 1024) return `${bytes} o`
-    if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(0)} Ko`
-    return `${(bytes / (1024 * 1024)).toFixed(1)} Mo`
-}
 </script>