Add tests for user data cleanup with shared document ownership and for TTS segment
variant export scenarios, including cases with duplicate audio keys and storage
disabled. Refactor cleanup logic to use document row types and transactional
ownership removal with last-owner checks. Update TTS segment cache clearing to
report deleted segment count by unique entry. Adjust export logic to always
include all TTS segment variants and conditionally export file buckets based on
storage availability. Update migration to use NOT VALID/VALIDATE for new
cascading constraints.