Add a workspace-leading partial index on ga_events for per-session path aggregation.

The query scans ga_events by workspace_id + inserted_at range, restricted to node-eligible rows (event_type IN ('pageview','lead','sale') AND session_id IS NOT NULL), then aggregates per session. Without this index the planner falls back to the global (event_type, inserted_at) index and applies workspace_id as a heap filter — scanning every tenant's events for the window. This partial, workspace-leading index serves the scan directly.