Add _VALID_EVENT_PREFIXES frozenset and validate event_type at all three
filter sites in audit.py (_build_filtered_query, _build_filtered_query
_with_handles, and the inline count query in list_audit_log). Invalid
values return HTTP 422. Also change like() pattern from prefix% to
prefix.% to enforce true prefix-match semantics.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>