feat: implement checksum-based duplicate detection for split email attachments

paperless-frontend/src/api/email-import.ts

@@ -57,6 +57,11 @@ export const emailImportApi = {
     return res.data;
   },
 
+  checkSplitChecksum: async (attachmentId: number, pages: { start: number; end: number }): Promise<boolean> => {
+    const res = await api.post<{ isDuplicate: boolean }>(`/api/email-import/attachments/${attachmentId}/check-split-checksum`, { pages });
+    return res.data.isDuplicate;
+  },
+
   executeImport: async (emailDate: string, attachments: AttachmentImportData[]): Promise<{ success: boolean; results: any[] }> => {
     const res = await api.post('/api/email-import/execute', { emailDate, attachments });
     return res.data;