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/alembic/versions/0002_add_document_title.py

@@ -0,0 +1,24 @@
+"""add document title column
+
+Revision ID: 0002
+Revises: 0001
+Create Date: 2026-04-14
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0002"
+down_revision: Union[str, None] = "0001"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("documents", sa.Column("title", sa.String(500), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("documents", "title")