feat(multi-user): add multi-user feature flag, owner_id model field, and user-scoped queries

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/versions/012_add_multi_user_support.py

@@ -0,0 +1,29 @@
+"""Add multi-user support: owner_id column on files table
+
+Revision ID: 012_add_multi_user_support
+Revises: 011_add_pdfa_paths
+Create Date: 2026-03-05
+
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "012_add_multi_user_support"
+down_revision: Union[str, None] = "011_add_pdfa_paths"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add owner_id column to files table for multi-user document isolation."""
+    op.add_column("files", sa.Column("owner_id", sa.String(), nullable=True))
+    op.create_index("ix_files_owner_id", "files", ["owner_id"])
+
+
+def downgrade() -> None:
+    """Remove owner_id column from files table."""
+    op.drop_index("ix_files_owner_id", table_name="files")
+    op.drop_column("files", "owner_id")