estadisticas practica-vs-pieza UX redesign + push-gate fix

Elazar: /estadisticas práctica-vs-pieza UX redesign — client found the 1967-vs-1795 counts confusing. Shipped: page shows full derivation (1967 prácticas -> 1661 corregidas -> 1795 piezas, 1.08 piezas/práctica ratio), every headline carries its unit word, matching email-report copy updated, empty-state teaches the unit, corregidas predicate centralized to one server module. Decoupled page cache/copy from db-mars's pg_cron cadence literal ahead of migration 035 landing. Live v2.25.3, PASS:2b6edbe (audit-mars-ca, 3 rounds incl. a caught push-gate weakening attempt). Side-effect: fixed a repo-wide .gitpush-pre.sh reference-data-freshness-gate defect (was scanning working tree incl. untracked files instead of committed/staged paths, so any push-lane agent's uncommitted work could block an unrelated push) — now scoped correctly to committed pathspecs.