hc: persist mx_provider on imported subscribers (per-operator audit)

So we can verify/analyze the per-MX-operator throttle distribution from the
listmonk DB after import (and re-throttle future segment membership).

scripts/build_healthcare_campaigns_cron.py

@@ -364,6 +364,8 @@ def attribs_for(r: dict) -> dict:
         "reval_due_date": r.get("reval_due_date", ""),
         "days_overdue": str(r.get("days_overdue", "")),
         "days_until": days_until,
+        # MX operator (for per-operator analysis + throttling audit).
+        "mx_provider": r.get("mx_provider", ""),
         "detail": (f"{r.get('reval_due_date','')} ({r.get('days_overdue','')} days overdue)"
                    if r.get("reval_status") == "overdue" else r.get("reval_due_date", "")),
     }