fix: stop idempotent job insert storms

[chubes4]

Summary

• bound job labels to the owned varchar(255) schema so oversized batch titles remain insertable
• recover duplicate races and lock/deadlock contention with bounded attempts while preserving the original write error
• rate-limit identical persistent error logs, emit an unthrottled metrics hook, and avoid exposing raw idempotency keys
• add focused regression coverage for malformed labels, race winners, transient contention, persistent failures, and log amplification

Production mechanism

Read-only Events diagnostics found one durable worklist item retried continuously with a 295-character title. Calling WordPress's wpdb::process_fields() against the live jobs schema returned false specifically for that label before SQL execution. The recovery lookup then reset wpdb->last_error, so every failure was logged with an empty database error and the worklist returned the item to ready for another attempt.

The existing unique idx_idempotency_key index is present and healthy; Action Scheduler retention pressure remains separately tracked in #2792 and #2793.

Verification

• php tests/jobs-idempotent-insert-recovery-smoke.php (21 assertions)
• Homeboy changed-file lint: PHPCS passed, PHPStan level 7 passed
• php tests/prefix-policy-audit.php (11 passed)
• Homeboy architecture audit: no introduced findings
• git diff --check

Homeboy's selected WP smoke wrapper could not launch because the host WP Codebox cache is missing packages/cli/dist/index.js; the selected standalone test passes directly.

Closes #3058