Replace mutation-lock with document-lock to centralize document mutation
serialization and database transaction management. Introduce runInDbTransaction
utility to abstract SQLite/Postgres transaction differences. Update document
deletion, user data cleanup, and rate limiter logic to delegate transaction
handling and locking to shared helpers. Remove dialect-specific branching and
inline transaction logic for improved maintainability and testability.
BREAKING CHANGE: withDocumentMutationLock is removed in favor of withDocumentLock and runInDbTransaction
