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/client-ticket/ProjectClientTickets.vue

@@ -211,7 +211,7 @@ const props = defineProps<{
 
 const { t } = useI18n()
 const clientTicketService = useClientTicketService()
-const { typeBadgeClass, statusBadgeClass, formatDate } = useClientTicketHelpers()
+const { typeBadgeClass, statusBadgeClass, formatDate, getAvailableStatusTransitions } = useClientTicketHelpers()
 
 const isOpen = ref(false)
 const isLoading = ref(false)
@@ -238,18 +238,7 @@ const isUpdatingStatus = ref(false)
 
 const availableStatusTransitions = computed(() => {
     if (!statusTarget.value) return []
-    const current = statusTarget.value.status
-    const allStatuses: { label: string; value: ClientTicketStatus }[] = [
-        { label: t('clientTicket.status.new'), value: 'new' },
-        { label: t('clientTicket.status.in_progress'), value: 'in_progress' },
-        { label: t('clientTicket.status.done'), value: 'done' },
-        { label: t('clientTicket.status.rejected'), value: 'rejected' },
-    ]
-    return allStatuses.filter(s => {
-        if (s.value === current) return false
-        if ((current === 'done' || current === 'rejected') && s.value === 'new') return false
-        return true
-    })
+    return getAvailableStatusTransitions(statusTarget.value.status, t)
 })
 
 async function loadTickets() {