## Why
Failure to open the diagnostic logs database should not disable thread storage or database-only history listing. Thread deletion must still preserve saved conversations when diagnostic log cleanup cannot complete.
## What changed
- Allow state initialization to continue after non-corruption errors opening the logs database, while preserving corruption recovery.
- Report the original diagnostic database error after tracing is installed in the TUI and app server, and return it from log operations.
- Reject thread deletion before destructive cleanup when diagnostic storage failed to initialize.
- Continue periodic cleanup of legacy logs when the current logs database is unavailable.
## Testing
Extend coverage for unavailable log directories, database-only history listing, startup diagnostics, deletion preserving saved conversations, corrupt log headers, and periodic legacy log cleanup during a diagnostic storage outage.
GitOrigin-RevId: a5184580ae6c39ed0c4eefa8cb48ee7731a991c3