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/composables/useClientTicketHelpers.ts

@@ -1,3 +1,5 @@
+import type { ClientTicketStatus } from '~/services/dto/client-ticket'
+
 export function useClientTicketHelpers() {
     function typeBadgeClass(type: string): string {
         switch (type) {
@@ -25,5 +27,22 @@ export function useClientTicketHelpers() {
         })
     }
 
-    return { typeBadgeClass, statusBadgeClass, formatDate }
+    function getAvailableStatusTransitions(
+        current: ClientTicketStatus,
+        t: (key: string) => string,
+    ): { label: string; value: ClientTicketStatus }[] {
+        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 { typeBadgeClass, statusBadgeClass, formatDate, getAvailableStatusTransitions }
 }