New types and helpers for context-aware DB routing and per-test transactions:
- PoolProxy: wraps pgxpool.Pool, routes queries through context transaction when active
- Querier: interface accepted by fixture/helper functions for decoupling
- ContextWithTx / TxFromContext: store/extract pgx.Tx in context.Context
- SetupTestTx (in testutils): begins tx, stores in context, auto-rolls back on cleanup
- SetupTestTx (in testtx): package-level variant with semaphore for parallel safety
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
