fix: prevent delete_all('') DB wipe, add thread safety, validate input types

- #255: Change `if user_id:` to `if user_id is not None:` with explicit
  ValueError for empty strings, preventing accidental full database wipe
- #256: Add threading.Lock around all write operations (add, delete,
  update, delete_all) to prevent data loss from concurrent writes
- #257: Add isinstance(content, str) check in add() to reject non-string
  types instead of silent SQLite coercion
- #243: Promote delete_all() cleanup failure logging from DEBUG to WARNING
  so orphaned rows in related tables are visible to operators

Fixes #255, #256, #257, #243

Author: buildingjoshbetter