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/stores/timer.ts

@@ -66,6 +66,11 @@ export const useTimerStore = defineStore('timer', () => {
         startTicking()
     }
 
+    function toIri<T extends { '@id'?: string; id: number }>(entity: T | string, prefix: string): string {
+        if (typeof entity === 'string') return entity
+        return entity['@id'] ?? `${prefix}/${entity.id}`
+    }
+
     async function startFromTask(task: Task) {
         const authStore = useAuthStore()
         if (!authStore.user) return
@@ -79,11 +84,9 @@ export const useTimerStore = defineStore('timer', () => {
             startedAt: new Date().toISOString(),
             user: `/api/users/${authStore.user.id}`,
             title: task.title,
-            project: task.project
-                ? (typeof task.project === 'string' ? task.project : (task.project['@id'] ?? (task.project.id ? `/api/projects/${task.project.id}` : null)))
-                : null,
-            task: typeof task === 'string' ? task : (task['@id'] ?? `/api/tasks/${task.id}`),
-            tags: task.tags?.map((t) => typeof t === 'string' ? t : (t['@id'] ?? `/api/task_tags/${t.id}`)) ?? [],
+            project: task.project ? toIri(task.project, '/api/projects') : null,
+            task: toIri(task, '/api/tasks'),
+            tags: task.tags?.map(t => toIri(t, '/api/task_tags')) ?? [],
         })
         startTicking()
     }