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/useApi.ts

@@ -29,13 +29,14 @@ export type ApiFetchOptions<ResponseType extends 'json' | 'blob'> =
     toastSuccessKey?: string
   }
 
-export const useApi = (): ApiClient => {
+let isHandlingUnauthorized = false
+
+export function useApi(): ApiClient {
   const config = useRuntimeConfig()
   const baseURL = config.public.apiBase || '/api'
   const toast = useToast()
   const auth = useAuthStore()
   const nuxtApp = useNuxtApp()
-  let isHandlingUnauthorized = false
   const i18n = nuxtApp.$i18n as
     | {
         t: (key: string) => string
@@ -45,7 +46,7 @@ export const useApi = (): ApiClient => {
   const t = (key: string) => (i18n?.t ? String(i18n.t(key)) : key)
   const te = (key: string) => (i18n?.te ? i18n.te(key) : false)
 
-  const extractErrorMessage = (error: unknown, responseData?: unknown): string => {
+  function extractErrorMessage(error: unknown, responseData?: unknown): string {
     const data = responseData ?? (error as FetchError)?.data
 
     if (typeof data === 'string') {
@@ -169,11 +170,11 @@ export const useApi = (): ApiClient => {
     }
   })
 
-  const request = <T>(
+  function request<T>(
     method: 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE',
     url: string,
     options: ApiFetchOptions<'json'> = {}
-  ) => {
+  ) {
     const needsJsonBody = method === 'POST' || method === 'PUT'
     const needsMergePatch = method === 'PATCH'