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/auth.ts

@@ -1,22 +1,22 @@
 import type { UserData } from './dto/user-data'
 
-export const getCurrentUser = () => {
-  const api = useApi()
-  return api.get<UserData>('/me', {}, { toastErrorKey: 'errors.auth.session' })
+export function getCurrentUser() {
+    const api = useApi()
+    return api.get<UserData>('/me', {}, { toastErrorKey: 'errors.auth.session' })
 }
 
-export const login = (username: string, password: string) => {
-  const api = useApi()
-  return api.post('/login_check', { username, password }, {
-    toastOn401: true,
-    toastErrorKey: 'errors.auth.login'
-  })
+export function login(username: string, password: string) {
+    const api = useApi()
+    return api.post('/login_check', { username, password }, {
+        toastOn401: true,
+        toastErrorKey: 'errors.auth.login'
+    })
 }
 
-export const logout = () => {
-  const api = useApi()
-  return api.post('/logout', {}, {
-    toastErrorKey: 'errors.auth.logout',
-    toastSuccessKey: 'success.auth.logout'
-  })
+export function logout() {
+    const api = useApi()
+    return api.post('/logout', {}, {
+        toastErrorKey: 'errors.auth.logout',
+        toastSuccessKey: 'success.auth.logout'
+    })
 }