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/models/document.py

@@ -16,6 +16,7 @@ class Document(Base):
     file_path: Mapped[str] = mapped_column(String, nullable=False)
     file_size: Mapped[int] = mapped_column(Integer, nullable=False)
     status: Mapped[str] = mapped_column(String, nullable=False, default="pending")
+    title: Mapped[str | None] = mapped_column(String(500), nullable=True)
     document_type: Mapped[str | None] = mapped_column(String, nullable=True)
     raw_text: Mapped[str | None] = mapped_column(Text, nullable=True)
     extracted_data: Mapped[str | None] = mapped_column(Text, nullable=True)  # JSON string