Use uuid.UUID everywhere and have postgres generate ids

postgres/queries/transactions.sql

@@ -0,0 +1,9 @@
+-- name: CreateTransaction :one
+INSERT INTO transactions
+(budget_id, date, memo, amount)
+VALUES ($1, $2, $3, $4)
+RETURNING *;
+
+-- name: GetTransactionsForBudget :many
+SELECT transactions.* FROM transactions 
+WHERE transactions.budget_id = $1;