editarPractica insert-fresh loses selfRegisteredAt + accumulates soft-del participation history

Follow-up flagged during VENUS-172. editarPractica soft-deletes active participants then INSERTs fresh (partial index excludes soft-deleted), so (a) selfRegisteredAt resets to NULL when a student's self-added participation is rewritten by a staff edit, (b) soft-deleted participation rows accumulate per edit. NOT a regression — old resurrect-in-place code never ran (always 500'd pre-VENUS-172). Fix if product wants selfRegisteredAt preserved across edits: resurrect-in-place via explicit UPDATE-then-INSERT. Deferred pending Elazar product call.