sqldb+invoices: Optimize invoice fetching when the reference is only a hash

The current sqlc GetInvoice query experiences incremental slowdowns during
the migration of large invoice databases, primarily due to its complex
predicate set. For this specific use case, a streamlined GetInvoiceByHash
function provides a more efficient solution, maintaining near-constant
lookup times even with extensive table sizes.

sqldb/sqlc/queries/invoices.sql

@@ -54,6 +54,11 @@ WHERE (
 GROUP BY i.id
 LIMIT 2;
 
+-- name: GetInvoiceByHash :one
+SELECT i.*
+FROM invoices i
+WHERE i.hash = $1;
+
 -- name: GetInvoiceBySetID :many
 SELECT i.*
 FROM invoices i