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/components/task/TaskModal.vue

@@ -154,7 +154,7 @@
                         />
                         <TaskDocumentList
                             v-if="isEditing && task"
-                            :documents="documents"
+                            :documents="localDocuments"
                             :is-admin="isAdmin"
                             @preview="openPreview"
                             @delete="handleDeleteDocument"
@@ -164,7 +164,7 @@
                         <TaskDocumentPreview
                             :document="previewDoc"
                             :has-prev="previewIndex > 0"
-                            :has-next="previewIndex < documents.length - 1"
+                            :has-next="previewIndex < localDocuments.length - 1"
                             @close="previewDoc = null"
                             @prev="prevPreview"
                             @next="nextPreview"
@@ -396,6 +396,14 @@ watch(() => props.modelValue, async (open) => {
         } catch {
             clientTickets.value = []
         }
+        if (props.task?.project?.giteaOwner && props.task?.project?.giteaRepo && !giteaUrl.value) {
+            try {
+                const settings = await getGiteaSettings()
+                giteaUrl.value = settings.url ?? ''
+            } catch {
+                // Gitea not available
+            }
+        }
     }
 })
 
@@ -405,17 +413,6 @@ watch(() => props.task, (task) => {
     }
 })
 
-watch(() => props.modelValue, async (open) => {
-    if (open && props.task?.project?.giteaOwner && props.task?.project?.giteaRepo && !giteaUrl.value) {
-        try {
-            const settings = await getGiteaSettings()
-            giteaUrl.value = settings.url ?? ''
-        } catch {
-            // Gitea not available
-        }
-    }
-})
-
 const { create, update, remove } = useTaskService()
 const { remove: removeDocument, getByTask: getDocumentsByTask } = useTaskDocumentService()
 const clientTicketService = useClientTicketService()
@@ -440,7 +437,6 @@ function ticketStatusClass(status: string): string {
 }
 
 const localDocuments = ref<TaskDocument[]>([])
-const documents = computed(() => localDocuments.value)
 const previewDoc = ref<TaskDocument | null>(null)
 
 // Sync documents from task prop when modal opens or task changes
@@ -455,7 +451,7 @@ async function refreshDocuments() {
 
 const previewIndex = computed(() => {
     if (!previewDoc.value) return -1
-    return documents.value.findIndex(d => d.id === previewDoc.value!.id)
+    return localDocuments.value.findIndex(d => d.id === previewDoc.value!.id)
 })
 
 function openPreview(doc: TaskDocument) {
@@ -464,13 +460,13 @@ function openPreview(doc: TaskDocument) {
 
 function prevPreview() {
     if (previewIndex.value > 0) {
-        previewDoc.value = documents.value[previewIndex.value - 1]
+        previewDoc.value = localDocuments.value[previewIndex.value - 1]
     }
 }
 
 function nextPreview() {
-    if (previewIndex.value < documents.value.length - 1) {
-        previewDoc.value = documents.value[previewIndex.value + 1]
+    if (previewIndex.value < localDocuments.value.length - 1) {
+        previewDoc.value = localDocuments.value[previewIndex.value + 1]
     }
 }