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/TaskDocumentPreview.vue

@@ -56,7 +56,7 @@
                     <div v-else class="flex flex-col items-center gap-4 rounded-xl bg-white p-10">
                         <Icon name="heroicons:document" class="h-16 w-16 text-neutral-400" />
                         <p class="max-w-xs truncate text-lg font-medium text-neutral-700">{{ document.originalName }}</p>
-                        <p class="text-sm text-neutral-400">{{ formatSize(document.size) }}</p>
+                        <p class="text-sm text-neutral-400">{{ formatFileSize(document.size) }}</p>
                         <a
                             :href="downloadUrl"
                             download
@@ -77,6 +77,7 @@
 <script setup lang="ts">
 import type { TaskDocument } from '~/services/dto/task-document'
 import { useTaskDocumentService } from '~/services/task-documents'
+import { formatFileSize } from '~/utils/format'
 
 const props = defineProps<{
     document: TaskDocument | null
@@ -98,12 +99,6 @@ const downloadUrl = computed(() => props.document ? getDownloadUrl(props.documen
 const isImage = computed(() => props.document?.mimeType.startsWith('image/') ?? false)
 const isPdf = computed(() => props.document?.mimeType === 'application/pdf')
 
-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`
-}
-
 // Focus overlay for keyboard events
 watch(() => props.document, (doc) => {
     if (doc) {