Restore DATABASE_URL in heartbeat runtime for Step 6 schema-drift sentinel

Step 6 schema-drift check has been not_runnable for 3+ consecutive days (June 1-3) because DATABASE_URL is not set in the heartbeat runtime environment. Decision: Either (a) add DATABASE_URL to heartbeat env so the schema-drift sentinel query can run, OR (b) mark schema-drift as NOT_YET_IMPLEMENTED in HEARTBEAT.md Step 6 if we decide not to run this check. This was flagged in the May 31 Sunday audit.