feat(canopy-medicaid): dead-letter on missing person_ids in tanf.case_closed subscriber

Source: tma-subscriber-person-lookup plan Potential Improvements.

The TMA subscriber currently logs and skips when person_ids is absent from tanf.case_closed payloads. Loud failure (dead-letter) would surface publisher regressions instead of silently dropping events. This becomes worthwhile once a second program subscribes to the same payload and benefits from the per-person list.

Acceptance: subscribe with DLX wired to canopy.dlq.tma-coverage; events without person_ids route to DLQ instead of being skipped; alerting on DLQ depth via Prometheus.