Fix audit log migration: entity_snippet deploy failure

The Audit log now correctly stamps a plain-text preview (entity_snippet) on activity events for untitled memories, and the four memory write operations route through the shared activity logger. This fixes the deploy failure from #429 caused by PostgreSQL rejecting a CREATE OR REPLACE FUNCTION that changed the return type of memsprout.audit_log — the migration has been corrected to DROP the old function first, then CREATE the new one.