Add transaction support

[erikgaas]

Changes

• Database.transaction() context manager — acquires a single connection from the pool, starts a transaction, and uses a contextvars.ContextVar to route all queries through it. Existing code (db.t.table.insert(...), db.q(...), etc.) automatically participates in the transaction with no changes needed.

• Fix _ColsGetter.__call__ — was referencing self.tbl.columns which doesn't exist on Table. Changed to self.tbl.cols.

Usage

async with db.transaction():
    await db.t.credit.update(...)
    await db.execute('UPDATE usage SET billed=true ...')
    # if either fails, both roll back

How it works

A contextvars.ContextVar (_txn_conn) holds the active transaction connection. Database.__getattr__ checks it before falling back to self.conn:

def __getattr__(self, k): return getattr(_txn_conn.get() or self.conn, k)

This means zero changes to calling code — everything just works inside an async with db.transaction() block.

Tests

• Rollback test: insert + raise → verify count unchanged
• Commit test: insert → verify count increased → cleanup

[erikgaas]

Bugfix. .columns returns a dict. cols returns a list of the names.