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>

features/doc-service/app/schemas/document.py

@@ -13,6 +13,7 @@ class DocumentOut(BaseModel):
     id: str
     user_id: str
     filename: str
+    title: str | None
     file_size: int
     status: str
     document_type: str | None
@@ -41,3 +42,7 @@ class DocumentTypeUpdate(BaseModel):
 
 class TagsUpdate(BaseModel):
     tags: list[str]
+
+
+class TitleUpdate(BaseModel):
+    title: str