GA4/UA importer missing — biggest Fathom-migration blocker

Severity: High (parity blocker) · Where: n/a (no importer exists; data handlers are export-only) Migrating users start from zero history. Fathom/Plausible offer GA import. Fix: Add a GA4 (Data API) / UA importer that backfills historical pageviews/sessions into the rollup store.

Phase A shipped (bdefab9): GA4/UA CSV import → rollup store. Auto-detects traffic/pages/sources/devices/browsers/os/geo/events files by header, never overwrites tracked days, zero-fills gaps for contiguous-prefix reads, batched writes (multi-year import in seconds), Settings → Manage UI. 8 E2E tests + live-verified. Remaining for Phase B: GA4 Data API connector with service-account JSON key (no OAuth) for automatic full-history pulls.