feat!: _async() twin support for methods that actually reach the database

[robinvandernoord]

Breaking Changes

• caching: save_to_cache() and memoize() no longer commit as a side effect of writing
  cache entries. Cache writes now join the caller's transaction, so they persist only when the
  caller commits. This is breaking because code that relied on cache entries being durable
  without an explicit db.commit() may now see those entries rolled back or missing.
• async safety: synchronous database statements run from a thread with a running event
  loop now emit BlockingDatabaseAccessWarning by default. This is breaking because existing
  async applications that used blocking ORM calls, lazy relationships, update_record(), or raw
  executesql() previously worked silently, and those patterns can now warn or fail where
  warnings are treated as errors.

Features

• Add an async execution layer with *_async twins for query, table, row, and rows
  operations.
• Add db.session() as an async transaction context manager, plus db.run_sync(),
  commit_async(), rollback_async(), executesql_async(), and close_async().
• Add row mutation twins: insert_async(), update_async(), update_record_async(),
  delete_async(), and delete_record_async().
• Add async_workers configuration, defaulting to max(4, pool_size), to bound the async
  worker and connection pool.
• Export AsyncSession, BlockingAccessHandler, and BlockingDatabaseAccessWarning.

Fixes

• Settle failed async transactions by committing or rolling back before releasing a worker.
• Prevent async worker leaks when tasks are cancelled or abandoned.
• Include the executed SQL command in blocking-access warnings.

Documentation

• Document async usage, transactions, lazy-loading limits, blocking-access warnings, and
  async_workers.