added IF NOT EXISTS to create and CRUD for each table

backend/database/sql/queries/audio_files.sql

@@ -0,0 +1,17 @@
+-- name: CreateAudioFile :one
+INSERT INTO audio_files (file_path, length_milliseconds, file_type_id, recording_id) VALUES (?, ?, ?, ?)
+RETURNING *;
+
+-- name: GetAudioFile :one
+SELECT * FROM audio_files 
+WHERE id = ? LIMIT 1;
+
+-- name: UpdateAudioFile :exec
+UPDATE audio_files 
+SET file_path = ?, length_milliseconds = ?, file_type_id = ?, recording_id = ?
+WHERE id =?;
+
+-- name: DeleteAudioFile :exec
+DELETE FROM audio_files 
+WHERE id =?;
+