practicaCodigo format: add PRA-MARS- prefix

Endodoncia I quote item 2 (accepted 2026-08-03, simplified 2026-08-05). Change practicaCodigo format to add a static PRA-MARS- prefix: PRA-MARS-YYMMDD-## instead of bare YYMMDD-##. Keeps the existing per-student-per-day sequence generated in practicas/nueva/actions.ts (~line 720). Confirmed by grep: nothing in the codebase parses/slices/matches practicaCodigo — every other site (search ILIKE, display, notification body, changelog) treats it as an opaque string, so this is a display-format-only change, one call site to touch. numeroHistoriaClinica is explicitly untouched — separate governance question tracked in MARS-534/MARS-331.