[DB] WithTx context/transaction helper

[cssbruno]

Add transaction helper support around database/sql.

Scope:

• Provide a WithTx-style helper that passes context and rolls back on errors.
• Avoid domain-specific repository abstractions.

Acceptance criteria:

• Commit and rollback behavior is tested.
• Context cancellation behavior is documented or tested.
• Docs show usage without adding a schema DSL.