feat: integrate platform backend and application interfaces

server/migrations/0019_v1_message_idempotency.sql

@@ -0,0 +1,6 @@
+ALTER TABLE IF EXISTS email_sms_outbox
+  ADD COLUMN IF NOT EXISTS idempotency_key TEXT;
+
+CREATE UNIQUE INDEX IF NOT EXISTS email_sms_outbox_idempotency_idx
+  ON email_sms_outbox(idempotency_key)
+  WHERE idempotency_key IS NOT NULL;