Adds nested exception handling in add_files() and _migrate_fts_to_external()
to catch and log rollback failures. Uses exception chaining to preserve both
transaction and rollback errors, preventing silent database inconsistency.
Solution-ID: SOL-1735385400010
Issue-ID: ISS-1766921318981-10
Task-ID: T1
Add fallback validation to detect dead threads missed by
threading.enumerate(), ensuring all stale connections are cleaned.
Solution-ID: SOL-1735392000002
Issue-ID: ISS-1766921318981-3
Task-ID: T2