test: inject dead-letter persistence failures

[nooby6]

Summary

• Inject a PostgreSQL failure when writing a dead-letter record.
• Verify retry exhaustion rolls back the job state change when dead-letter persistence fails.
• Verify expired-lease recovery rolls back atomically when the dead-letter write fails.
• Update the README to distinguish completed failure-injection coverage from remaining process-level restart testing.

Why

A job must not be left marked failed without its corresponding dead-letter record. These tests exercise the transactional guarantee by forcing the database write to fail.

Validation

GitHub Actions will run the Go suite against PostgreSQL 16 and the claim benchmark.