Switch SMS billing to credit units with IRT top-ups.

Balance is SMS credits (1 per 64 chars); top-ups add floor(IRT/500) credits, track sms_sent_count, and convert existing IRT balances.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/085_business_sms_sent_count.sql

@@ -0,0 +1,8 @@
+-- Lifetime count of SMS messages successfully billed to each business
+-- (incremented on charge, decremented on refund). One unit = one recipient.
+
+ALTER TABLE businesses
+  ADD COLUMN IF NOT EXISTS sms_sent_count INTEGER NOT NULL DEFAULT 0;
+
+COMMENT ON COLUMN businesses.sms_sent_count IS
+  'Lifetime SMS messages billed to this business (one per recipient). Incremented on charge, decremented on refund.';