db: use a binary seaweedfs_fid

migrations/1_up.sql

@@ -1,6 +1,6 @@
 CREATE TABLE media(
 	media_id INTEGER PRIMARY KEY,
-	seaweedfs_fid TEXT (33) NOT NULL,
+	seaweedfs_fid BLOB (32) NOT NULL,
 	filename TEXT (256) NOT NULL,
 	content_type TEXT (256)
 );