- Fix duplicate SSE event on comment creation: CreateMemoComment now
  suppresses the redundant memo.created broadcast from the inner
  CreateMemo call, emitting only memo.comment.created
- Extract reaction event-building IIFEs into buildMemoReactionSSEEvent
  helper, removing duplicated inline DB-fetch logic
- Promote resolveSSEAudienceCreatorID from method to free function
  (resolveSSECreatorID) since it never used the receiver
- Add userID to SSE connect/disconnect log lines for traceability
- Change canReceive default from permissive (return true) to
  deny-with-warning for unknown visibility types
- Add comprehensive tests covering all new helpers, visibility edge
  cases, slow-client drop behavior, and the double-broadcast fix
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>