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/services/task-documents.ts

@@ -15,30 +15,24 @@ export function useTaskDocumentService() {
         return extractHydraMembers(data)
     }
 
-    async function upload(taskId: number, file: File): Promise<TaskDocument> {
+    async function uploadWithRelation(relationField: string, relationIri: string, file: File): Promise<TaskDocument> {
         const formData = new FormData()
         formData.append('file', file)
-        formData.append('task', `/api/tasks/${taskId}`)
+        formData.append(relationField, relationIri)
 
-        return await $fetch<TaskDocument>(`${baseURL}/task_documents`, {
+        return $fetch<TaskDocument>(`${baseURL}/task_documents`, {
             method: 'POST',
             body: formData,
             credentials: 'include',
-            // Do NOT set Content-Type — browser sets multipart boundary automatically
         })
     }
 
-    async function uploadForTicket(clientTicketId: number, file: File): Promise<TaskDocument> {
-        const formData = new FormData()
-        formData.append('file', file)
-        formData.append('clientTicket', `/api/client_tickets/${clientTicketId}`)
+    async function upload(taskId: number, file: File): Promise<TaskDocument> {
+        return uploadWithRelation('task', `/api/tasks/${taskId}`, file)
+    }
 
-        return await $fetch<TaskDocument>(`${baseURL}/task_documents`, {
-            method: 'POST',
-            body: formData,
-            credentials: 'include',
-            // Do NOT set Content-Type — browser sets multipart boundary automatically
-        })
+    async function uploadForTicket(clientTicketId: number, file: File): Promise<TaskDocument> {
+        return uploadWithRelation('clientTicket', `/api/client_tickets/${clientTicketId}`, file)
     }
 
     async function getByTicket(clientTicketId: number): Promise<TaskDocument[]> {