sqldb+invoices: add migration to fix incorrectly stored invoice expiries

Previously, when using the native schema, invoice expiries were incorrectly
stored as 64-bit values (expiry in nanoseconds instead of seconds), causing
overflow issues. Since we cannot determine the original values, we will set
the expiries for existing invoices to 1 hour with this migration.

sqldb/sqlc/migrations/000004_invoice_expiry_fix.down.sql

@@ -0,0 +1,2 @@
+-- Given that all expiries are changed in this migration we won't be able to
+-- roll back to the previous values.