Support multi-tenant session namespacing (user_id/project_id) #23

To simplify integration into multi-tenant backend products, the session schema and SDK should support first-class namespacing.

Requirements:

• Add user_id and project_id as first-class fields in the Session schema.
• Update the SDK to allow filtering and managing sessions by these namespaces.
• Ensure the database indexes support efficient querying by these new identifiers.