reorder_signals grew to 4.49M rows / 1.19GB — the compute job INSERTed a fresh
row per transceiver every 4h run but never deleted old ones (24h TTL filtered
them at read time via DISTINCT ON + expires_at, but they were never purged).
4.37M rows were already expired dead weight.
Fix: DELETE existing rows for a transceiver before inserting its new signal, so
the table holds exactly one (latest) row per transceiver. Cleaned up to 18,175
rows / 4.5MB (99.6% reclaimed, VACUUM FULL). Backup: reorder_signals_keep_bak_20260606.
Verified: re-running compute:reorder-signals keeps count stable at 18,175.