feat(core): add attachment API hooks

frontend/src/api/hooks.ts

@@ -725,3 +725,59 @@ export function useCreateSequence() {
     },
   });
 }
+
+// Attachments
+export interface Attachment {
+  id: string;
+  entity_type: string;
+  entity_id: string;
+  filename: string;
+  file_path: string;
+  mime_type: string;
+  file_size: number;
+  uploaded_by?: string | null;
+  created_at?: string;
+  updated_at?: string;
+}
+
+export function useAttachments(entityType?: string, entityId?: string) {
+  return useQuery({
+    queryKey: ['attachments', entityType, entityId],
+    queryFn: () => {
+      const params = new URLSearchParams();
+      if (entityType) params.set('entity_type', entityType);
+      if (entityId) params.set('entity_id', entityId);
+      return apiGet<{ items: Attachment[]; total: number }>(`/attachments?${params.toString()}`);
+    },
+    enabled: !!entityType && !!entityId,
+  });
+}
+
+export function useUploadAttachment() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: async ({ file, entityType, entityId }: { file: File; entityType: string; entityId: string }) => {
+      const formData = new FormData();
+      formData.append('file', file);
+      formData.append('entity_type', entityType);
+      formData.append('entity_id', entityId);
+      const response = await apiClient.post('/attachments', formData, {
+        headers: { 'Content-Type': 'multipart/form-data' },
+      });
+      return response.data;
+    },
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['attachments'] });
+    },
+  });
+}
+
+export function useDeleteAttachment() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: string) => apiDelete(`/attachments/${id}`),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['attachments'] });
+    },
+  });
+}