sqldb: add missing index for settled invoices

sqldb/sqlc/migrations/000008_invoice_add_settled_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS invoices_settle_index_idx;

sqldb/sqlc/migrations/000008_invoice_add_settled_index.up.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS invoices_settle_index_idx ON invoices(settle_index);