feat(postgres): data migration + per-BU closed counts in frontend
- Create backend/scripts/migrate-to-postgres.js (one-time SQLite→Postgres copy)
- Successfully migrated: 6 users, 21 CVEs, 6307 findings, 20965 compliance items,
138 archives, 67 atlas plans, all notes/overrides merged
- All 22 tables verified with matching row counts
- Frontend StatusDonut now uses server-provided per-BU counts (no more N/A)
- Counts endpoint called with teams param on scope change
- Re-fetch counts when admin scope toggle changes