Revise user data cleanup logic to ensure proper cascading deletion of user-related
database rows and S3 objects, including shared document and preview artifacts.
Introduce explicit checks for last ownership before removing shared resources.
Add TTS segment cache and audio cleanup to document and user deletion flows.
Expand user data export to include TTS segment entries and audio, job events,
document settings, and linked auth sessions. Update schema with ON DELETE CASCADE
for userTtsChars and userJobEvents. Add new migration scripts and comprehensive
unit tests for cleanup and export scenarios.
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