- Add TerminalSessionModel DB table with instance_id FK, name, status,
created_at, last_activity_at, closed_at columns
- Add Alembic migration for terminal_sessions table
- Refactor TerminalManager to use composite key (instance_id, session_id)
supporting up to 5 concurrent sessions per instance
- Add create_session, get_session, get_sessions_for_instance, close_session
- Preserve get_or_create_session for backward compatibility (default session)
- Fix attach_websocket to only close sockets within same session
- Add name (auto-generated 'Session N') and status tracking to TerminalSession
- Add 7 unit tests for multi-session logic
Quality gates: pytest (7 new passed, 174 total passed, 51 pre-existing failures)
