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

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+-- 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.';

database/migrations/086_sms_balance_credit_units.sql

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+-- Convert prepaid SMS balance from IRT to credit units.
+-- Previous billing: 400 IRT per 64-char segment.
+-- New model: balance is SMS credits (1 credit = 1 segment); top-up at 500 IRT/credit.
+-- Preserve existing prepaid capacity: IRT / 400 → credits.
+
+UPDATE businesses
+SET sms_balance = sms_balance / 400
+WHERE sms_balance > 0;
+
+COMMENT ON COLUMN businesses.sms_balance IS
+  'Prepaid SMS credit units. 1 unit = one 64-character segment per recipient. Top-up adds floor(IRT / 500) credits.';