Add AI-suggested editable document title

AI now returns a short descriptive title per document (e.g. "ACME Corp
Invoice April 2026"). Title is stored in a new documents.title column
(migration 0002), shown in the row header instead of the raw filename,
and editable inline via PATCH /documents/{id}/title. Filename is shown
as a subtitle when a title exists.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -86,6 +86,7 @@ export interface DocumentOut {
   id: string;
   user_id: string;
   filename: string;
+  title: string | null;
   file_size: number;
   status: DocumentStatus;
   document_type: string | null;
@@ -148,6 +149,9 @@ export const viewDocument = async (id: string): Promise<void> => {
 export const updateDocumentTags = (id: string, tags: string[]) =>
   api.patch<DocumentOut>(`/documents/${id}/tags`, { tags }).then((r) => r.data);
 
+export const updateDocumentTitle = (id: string, title: string) =>
+  api.patch<DocumentOut>(`/documents/${id}/title`, { title }).then((r) => r.data);
+
 export const assignCategory = (docId: string, catId: string) =>
   api.post(`/documents/${docId}/categories/${catId}`);