Add Database Transaction Handling Utilities for pgx and sqlc

[dytsou]

Type of changes

• Feature

Purpose

• Provide reusable transaction lifecycle helpers for pgx and sqlc call sites
• Reduce boilerplate around begin, commit, rollback, and WithTx rebinding

Additional Information

• Adds WithTransaction / TxRunner for raw pgx.Tx callbacks; reuses an existing tx when db is already pgx.Tx
• Adds WithTransactionQueries / QueriesTx[T] to bind sqlc-generated queries to the active transaction via Run and RunWithDBTx
• Includes unit tests in pkg/database/tx_test.go and pkg/database/tx_queries_test.go
• Bumps github.com/jackc/pgx/v5 from v5.7.4 to v5.9.2

Usage examples:

// sqlc-only callback
err := databaseutil.WithTransactionQueries(ctx, pool, logger, queries).
    Run(func(qtx *db.Queries) error {
        return qtx.UpdateUser(ctx, params)
    })

// callback needs both pgx.Tx and sqlc queries
err := databaseutil.WithTransactionQueries(ctx, pool, logger, queries).
    RunWithDBTx(func(tx pgx.Tx, qtx *db.Queries) error {
        return otherService.DoWork(ctx, tx, qtx)
    })

[yukicoder0509]

It seems this pull request introduces new transaction-helper APIs. I think we should document the detailed usage in README.